Thanks to all contributors, you're awesome and wouldn't be possible without you! The goal is to build a categorized community-driven collection of very well-known resources.
Algorithmia - Algorithm and data-structure library for .NET 3.5 and up. Algorithmia contains sophisticated algorithms and data-structures like graphs, priority queues, command, undo-redo and more.
API
NancyFx - Lightweight, low-ceremony, framework for building HTTP based services on .Net and Mono
ASP.NET WebAPI - Framework that makes it easy to build HTTP services that reach a broad range of clients, including browsers and mobile devices
Breeze - Api framework enabling rich data access by using the OData 3 protocol. Client libraries available for Javascript and C#.
Mobius: C# API for Spark - Mobius adds C# language binding to Apache Spark, enabling the implementation of Spark driver code and data processing operations in C#.
ServiceStack - Thoughtfully architected, obscenely fast, thoroughly enjoyable web services for all
Nelibur - Nelibur is message based web service framework on the pure WCF. Nelibur simplifies creating high-performance and message based web services and you certainly have all the power of the WCF.
WebAPI Contrib - Collection of open source projects to help improve your work with ASP.NET Web API
Application Frameworks
ASP.NET Boilerplate - A starting point for new modern ASP.NET MVC web applications with best practices and most popular tools.
Orleans - Orleans is a framework that provides a straight-forward approach to building distributed high-scale computing applications, without the need to learn and apply complex concurrency or other scaling patterns
CoreFX - The corefx repo contains the library implementation (called "CoreFX") for .NET Core. It includes System.Collections, System.IO, System.Xml and many other components. It builds and runs on Windows. You can 'watch' the repo to see Linux and Mac support being added over the next few months.
Mono - Mono open source ECMA CLI, C# and .NET implementation
Mono-Addins - Mono.Addins is a generic framework for creating extensible applications, and for creating add-ins which extend those applications
peasy - Peasy is a middle tier framework that offers an easy to use and flexible rules engine and was designed to address common challenges such as concurrency handling, transactional support, fault tolerance, threading, scalability, async and multiple client support, and easy testability, all without a huge learning curve!
Spring.Net - Spring.NET is an open source application framework that makes building enterprise .NET applications easier
DotNetty - DotNetty is a port of Netty, asynchronous event-driven network application framework for rapid development of maintainable high performance protocol servers & clients.
Application Templates
ASP.NET Core Starter Kit - backend: .NET Core, EF Core, C#; frontend: Babel, Webpack, React, CSS Modules
MVC.Template - ASP.NET Core MVC project starter template.
ProjectScaffold - A prototypical .NET solution recommended by the F# Foundation---includes file system setup, Paket for dependencies and FAKE for build/test automation. By default, build process also compiles documentation and generates NuGet packages.
Serene - Serenity is an ASP.NET MVC application platform designed to simplify and shorten development of data-centric business applications with a service based architecture. Serene is a starter template to build Serenity applications.
Side-Waffle - Large collection of useful templates for Web and Desktop development.
Template10 - Windows 10 templates with design patterns.
Artificial Intelligence
AIMLBot (Program#) - A small, fast, standards-compliant yet easily customizable implementation of an AIML (Artificial Intelligence Markup Language) based chatter bot in C#.
SIML - Synthetic Intelligence Markup Language, a next generation Chatbot & Digital Assistant Language.
Assembly Manipulation
dnSpy - dnSpy is a .NET assembly editor, decompiler, and debugger forked from ILSpy.
Fody - Extensible tool for weaving .net assemblies
Mono.Cecil - Cecil is a library to generate and inspect programs and libraries in the ECMA CIL form.
Assets
Cassette - Manages .NET web application assets (scripts, css and templates)
NodeAssets - Asset Manager for .net that has live css updates using SignalR and can optionally leverage NodeJS compilers
CacheCow - An ASP.NET Web API HTTP caching implementation both on client and server
Akavache - An asynchronous, persistent key-value store
CacheManager - A common interface and abstraction layer for caching.
CacheAdapter - Another common cache abstraction layer for caching with enhanced cache API usage.
Foundatio - A common interface with in memory, redis and hybrid implementations.
CLI
Command Line Parser - The Command Line Parser Library offers to CLR applications a clean and concise API for manipulating command line arguments and related tasks
Fluent Command Line Parser - A simple, strongly typed .NET C# command line parser library using a fluent easy to use interface
Power Args - PowerArgs converts command line arguments into .NET objects that are easy to program against. It also provides a ton of optional capabilities such as argument validation, auto generated usage, tab completion, and plenty of extensibility
Argu - Declarative CLI argument & XML configuration parser for F# applications.
Appccelerate - Command Line Parser - A command line parser with fluent definition syntax, different argument types, required and optional arguments, value restrictions, aliases, type conversion and semi-automatic usage help message composition
CLR
CoreCLR - The coreclr repo contains the complete runtime implementation (called "CoreCLR") for .NET Core. It includes RyuJIT, the .NET GC, native interop and many other components. It builds and runs on Windows. You can 'watch' the repo to see Linux and Mac support being added over the next few months.
CMS
Composite C1 - A web CMS that focus on UX and adaptability
mojoPortal - MojoPortal is an extensible, cross database, mobile friendly, web content management system (CMS) and web application framework written in C# ASP.NET
N2CMS - Open source, lightweight, code-first CMS able to seamlessly integrate into any MVC project.
Orchard - Free, open source, community-focused project aimed at delivering applications and reusable components on the ASP.NET platform
Piranha CMS - Piranha is the fun, fast and lightweight .NET framework for developing cms-based web applications with an extra bite. It's built on ASP.NET MVC and Web Pages and is fully compatible with both Visual Studio and WebMatrix. http://piranhacms.org
Umbraco - Umbraco is a free open source Content Management System built on the ASP.NET platform
BetterCMS - Open Source .NET Intuitive User Interface for Developer and Publisher
DotNetNuke - DNN Platform is our free, open source web CMS and the foundation of every professional DNN solution. Over 750,000 organizations worldwide have built websites powered by the DNN Platform.
Code Analysis and Metrics
CodeMaid - Visual studio extension to cleanup, dig through and simplify C#, C++, F#, VB, PHP, JSON, XAML, XML, ASP, HTML, CSS, LESS, SCSS, JavaScript and TypeScript coding.
StyleCop - StyleCop analyzes C# source code to enforce a set of style and consistency rules
Gendarme - Extensible rule-based tool to find problems in .NET applications and libraries
Metrics-Net - Capturing CLR and application-level metrics. So you know what's going on.
AspNet.Metrics - Capturing CLR, application-level web request metrics. Middleware and extensions using Metrics-Net
Nemerle - Nemerle is a high-level statically-typed programming language for the .NET platform. It offers functional, object-oriented and imperative features. It has a simple C#-like syntax and a powerful meta-programming system. http://nemerle.org/About
Netjs - .NET to TypeScript and JavaScript compiler. Portable Class Libraries work great for this. You can even pass EXEs.
Roslyn - The .NET Compiler Platform ("Roslyn") provides open-source C# and Visual Basic compilers with rich code analysis APIs. It enables building code analysis tools with the same APIs that are used by Visual Studio.
LinqOptimizer - An automatic query optimizer-compiler for Sequential and Parallel LINQ
Compression
SharpCompress - SharpCompress is a compression library for .NET/Mono/Silverlight/WP7 that can unrar, un7zip, unzip, untar unbzip2 and ungzip with forward-only reading and file random access APIs. Write support for zip/tar/bzip2/gzip are implemented
Cassandra - DataStax .NET Driver for Apache Cassandra
Couchbase - Official couchbase .NET client library, based on the Enyim memcached client
Firebird.NET - The .NET Data provider is written in C# and provides a high-performance, native implementation of the Firebird API
Datetime
NodaTime - Noda Time is an alternative date and time API for .NET. It helps you to think about your data more clearly, and express operations on that data more precisely. http://nodatime.org/
DateTimeExtensions - Common Date Time operations on the form of extensions to System.DateTime, including holidays and working days calculations on several culture locales.
Exceptionless.DateTimeExtensions - DateTimeRange, Business Day and various DateTime, DateTimeOffset, TimeSpan extension methods.
Decompilation
ILSpy - ILSpy is the open-source .NET assembly browser and decompiler
Unfold - Powershell-based deployment solution for .net web applications
DbUp - .NET library that helps you to deploy changes to SQL Server databases. It tracks which SQL scripts have been run already, and runs the change scripts that are needed to get your database up to date
DirectX
SlimDX - DirectX framework wrapper for .NET applications
SharpDX - SharpDX is an open-source project delivering the full DirectX API for .Net on all Windows platforms, allowing the development of high performance game, 2D and 3D graphics rendering as well as realtime sound application.
Distributed Computing
Orleans - Orleans is a framework that provides a straight-forward approach to building distributed high-scale computing applications, without the need to learn and apply complex concurrency or other scaling patterns. It was created by Microsoft Research.
Orleankka - Orleankka is a functional API for Microsoft Orleans framework. It is highly suitable for scenarios where having composable, uniform communication interface is preferable, such as: CQRS, event-sourcing, re-routing, FSM, etc. Additional API avaiable for F# called Orleankka.FSharp.
Akka.net - Akka.NET is a port of the popular Java/Scala framework Akka to .NET. This is a community driven port and is not affiliated with Typesafe who makes the original Java/Scala version.
Zebus - Zebus is a lightweight, higlhy versatile, peer to peer service bus, built with CQRS principles in mind. It allows applications to communicate with each other in a fast and easy manner. Most of the complexity is hidden in the library and you can focus on writing code that matters to you, not debugging messaging code. A very fundamental base for any distributed application.
FsShelter - F# library for authoring Apache Storm components and topologies. Offering high-level abstractions for distributed and fault-tolerant event stream processing.
Foundatio - Pluggable foundation blocks for building distributed apps.
Virto Commerce - Virto Commerce is the second generation release and is the only enterprise level e-commerce product fully available under Open Source license. Virto Commerce is based on .NET 4.5 with extensive use of MVC, IoC, EF, Azure, Angular JS and many other cutting edge technologies. It can be deployed in Microsoft Cloud (Azure), Amazon Web Services (AWS) and on-premise. https://virtocommerce.com
Environment Management
Dotnet CLI - The cross-platform .NET Core command line toolchain utility.
ETL
Reactive ETL - Reactive ETL is a rewrite of Rhino ETL using the reactive extensions for .NET
Game
MonoGame - One framework for creating powerful cross-platform games
CocosSharp - CocosSharp is a C# implementation of the Cocos2D and Cocos3D APIs that runs on any platform where MonoGame runs
Duality - Duality is a 2D game development framework. Focused on modularity, comes with a visual editor.
Xenko - Xenko is a 2D/3D cross-platform game engine featuring a scene editor, particles, physically based rendering (PBR), scripting, and much more
Wave Engine - Wave engine is a free c# component-based modern game engine which allows you to create cross-platform games supporting kinect, oculusrift, vuforia, cardboard, leapmotion and much more. [Free][Proprietary]
UrhoSharp - UrhoSharp is a C# implementation of the Urho3D game engine that runs on iOS, Mac, Windows, Android and Linux systems
Nez - Nez is a free 2D focused framework that works with MonoGame and FNA
GIS
NetTopologySuite A .NET GIS solution that is fast and reliable for the .NET platform
SharpMap An easy-to-use mapping library for use in web and desktop applications
OsmSharp - C# library to work with OpenStreetMap (OSM) data. Provides reading, writing and route-planning for OSM data.
Git Tools
Bonobo Git Server - Bonobo Git Server for Windows is a web application you can install on your IIS and easily manage and connect to your git repositories. https://bonobogitserver.com/
Oxyplot - OxyPlot is a cross-platform plotting library for .NET
OpenTK - The Open Toolkit is an advanced, low-level C# library that wraps OpenGL, OpenCL and OpenAL
NGraphics - NGraphics is a cross-platform library for rendering vector graphics on .NET
Live-Charts - An Animated, MVVM oriented and modern charting library
GUI
MahApps.Metro - Toolkit for creating Metro styled WPF apps
Callisto - A control toolkit for Windows 8 XAML applications. Contains some UI controls to make it easier to create Windows UI style apps for the Windows Store in accordance with Windows UI guidelines.
ObjectListView - ObjectListView is a C# wrapper around a .NET ListView. It makes the ListView much easier to use and teaches it some new tricks
DockPanelSuite - The Visual Studio inspired docking library for .NET WinForms
AvalonEdit - The WPF-based text editor component used in SharpDevelop
XWT - A cross-platform UI toolkit for creating desktop applications with .NET and Mono
Gtk# - Gtk# is a Mono/.NET binding to the cross platform Gtk+ GUI toolkit and the foundation of most GUI apps built with Mono
Eto.Forms - Cross platform GUI framework for desktop and mobile applications in .NET and Mono
Dragablz - Dragable, tearable WPF tab control (similar to Chrome) which supports layouts and is full themeable, including themese compatible with MahApps and Material Design.
Fluent.Ribbon - Fluent Ribbon Control Suite is a library that implements an Office- and Windows 8-like Ribbon for WPF.
Office Ribbon - A library that implements MS Office Ribbon for WinForms.
MaterialSkin - Theming .NET WinForms, C# or VB.Net, to Google's Material Design Principles.
Xamarin.Forms - Build native UIs for iOS, Android and Windows from a single, shared C# codebase.
SciterSharp - Create .NET cross-platform desktop apps using not just HTML, but all features of Sciter engine: CSS3, SVG, scripting, AJAX, <video>... Sciter is free for commercial use
Empty Keys UI - Multi-platform and multi-engine XAML based user interface library [Free][Proprietary]
UWP Community Toolkit - The UWP Community Toolkit is a collection of helper functions, custom controls, and app services. It simplifies and demonstrates common developer tasks building UWP apps for Windows 10.
ScintillaNET - Windows Forms control for the Scintilla text editor component (Scintilla is used by Notepad++)
HTML and CSS
AngleSharp - Complete HTML5 DOM and CSS3 OM construction
CsQuery - HTML5 parser with jQuery style DOM interaction
TinyIoC - Single-file, easy and cross-platform IoC container
Simple Injector - Simple Injector is an easy-to-use Dependency Injection (DI) library for .NET 4+ that supports Silverlight 4+, Windows Phone 8, Windows 8 including Universal apps and Mono.
Scrutor - Assembly scanning extensions for Microsoft.Extensions.DependencyInjection.
JavaScript Engines
ClearScript - A library that makes it easy to add scripting to your .NET applications. It currently supports JavaScript (via V8 and JScript) and VBScript.
Edge.js - Run .NET and Node.js code in-process on Windows, MacOS, and Linux
Jint - JavaScript interpreter for .NET which provides full ECMA 5.1 compliance and can run on any .NET platform.
Jurassic - A implementation of the ECMAScript language and runtime. It aims to provide the best performing and most standards-compliant implementation of JavaScript for .NET.
Logging
Essential Diagnostics - Extends the inbuilt features of System.Diagnostics namespace to provide flexible logging
NLog - NLog - Advanced .NET and Silverlight Logging
Logary - Logary is a high performance, multi-target logging, metric, tracing and health-check library for mono and .Net. .Net's answer to DropWizard. Supports many targets, built for micro-services.
Log4Net - The Apache log4net library is a tool to help the programmer output log statements to a variety of output targets
Serilog - A no-nonsense logging library for the NoSQL era. Combines the best of traditional and structured diagnostic logging in an easy-to-use package.
Semantic Logging Application Block (SLAB) - Extends the inbuilt features of System.Diagnostics.Tracing namespace (EventSource class) to log to several sinks including Azure Tables, Databases, files (JSON, XML, text). Supports in-process and out-of-process logging through ETW, and Rx for real-time filtering/aggregating of events.
Foundatio - A fluent logging api that can be used to log messages throughout your application.
Loupe - Centralized .NET logging and monitoring. [Proprietary][Free Tier]
Machine Learning and Data Science
Infer.NET - A framework for running Bayesian inference in graphical models. It can also be used for probabilistic programming. [Proprietary][Free][Research]
Accord.NET - Machine learning framework combined with audio and image processing libraries (computer vision, computer audition, signal processing and statistics).
Accord.NET Extensions - Advanced image processing and computer vision algorithms made as fluent extensions.
AForge.NET - Framework for developers and researchers in the fields of Computer Vision and Artificial Intelligence (image processing, neural networks, genetic algorithms, machine learning, robotics).
Deedle - Data frame and (time) series library for exploratory data manipulation with C# and F# support
FsLab - A collection of data science and machine learning libraries for F# and .NET
numl - Designed to include the most popular supervised and unsupervised learning algorithms while minimizing the friction involved with creating the predictive models.
R Provider - Type provider that exposes R packages and functions in a type-safe way to F# callers
F# Data - F# type providers for accessing XML, JSON, CSV and HTML files (based on sample documents) and for accessing WorldBank data
Markdown Processors
MarkdownSharp - Open source C# implementation of Markdown processor, as featured on Stack Overflow.
F# Formatting - Tools for documenting F# and C# projects. The library contains extensible Markdown parser as a core component.
CommonMark.NET - Implementation of CommonMark specification in C# for converting Markdown documents to HTML. Optimized for maximum performance and portability.
markdig - A fast, powerful, CommonMark compliant, extensible Markdown processor for .NET.
Mail
FluentEmail - A Fluent Wrapper for System.Net.Mail with razor templating support.
MailKit - A complete cross-platform mail stack including IMAP, POP3, SMTP, authentication and more. Built on top of MimeKit.
MimeKit - A cross-platform .NET MIME creation and parser library with support for S/MIME, PGP, TNEF and Unix mbox spools.
PreMailer.Net - C# library that moves your stylesheets to inline style attributes, for maximum compatibility with E-mail clients.
Mathematics
MathNet - Math.NET is an opensource initiative to build and maintain toolkits covering fundamental mathematics, targetting advanced but also every day needs of .Net developers
Foundatio - A common interface with in memory, redis, StatsD, and Metrics.NET implementations.
Micro Framework
.NET Micro Framework Interpreter - Microsoft® .NET Micro Framework (NETMF) for developing embedded applications on small devices using Visual Studio
Minification
Microsoft Ajax Minifier - Contains JS and CSS minifiers which have a highest performance, because its have been specifically designed for .NET. Optionally produce Source Maps for JS code.
Web Markup Minifier - .NET library that contains a set of markup minifiers. The objective of this project is to improve the performance of web applications by reducing the size of HTML, XHTML and XML code.
Misc
.NET Fiddle - Write, compile and run C# code in the browser. The C# equivalent of JSFiddle.
CSharp Pad - A web based C# REPL with awesome code completion.
AzureCrawler - Take HTML Snapshots for your Angular, Ember, Durandal or any JavaScript applications
CSScript - CS-Script is a CLR based scripting system which usesC# as a programming language. CS-Script currently targets Microsoft implementation of CLR (.NET 2.0/3.0/3.5/4.0/4.5) with full support on Mono. Comes with many additional features, such as script hosting.
FlatMapper - A library to import and export data from and to plain text files in a Linq compatible way.
FluentValidation - A small validation library for .NET that uses a fluent interface and lambda expressions for building validation rules.
Humanizer - Humanizer meets all your .NET needs for manipulating and displaying strings, enums, dates, times, timespans, numbers and quantities
LINQPad - a C#/VB/F# scratchpad that instantly executes any expression, statement block or program with rich output formatting and a wealth of features. Also lets you interactively query databases in LINQ. [$]
Polly - Express transient exception handling policies such as Retry, Retry Forever, Wait andRetry or Circuit Breaker in a fluent manner. (.NET 3.5 / 4.0 / 4.5 / PCL / Xamarin)
ScriptCS - Write C# apps with a text editor, nuget and the power of Roslyn!
Shielded - Software Transactional Memory (STM) implementation for .NET
MSBuild ILMerge task - MSBuild ILMerge task is a NuGet package allows you to use the famous ILMerge utility in automated builds and/or Visual Studio projects.
ReactJS.NET - ReactJS.NET is a library that makes it easier to use Babel along with Facebook's React and JSX from C#.
Aeron.NET - Efficient reliable UDP unicast, UDP multicast, and IPC message transport - .NET port of Aeron
TypeShape - TypeShape is a small, extensible F# library for practical generic programming
Streams - A lightweight F#/C# library for efficient functional-style pipelines on streams of data.
MediatR - Simple, unambitious mediator implementation in .NET
Warden - Define "health checks" for your applications, resources and infrastructure. Keep your Warden on the watch
MVVM
Caliburn.Micro - A small, yet powerful framework, designed for building applications across all XAML platforms. Its strong support for MV* patterns will enable you to build your solution quickly, without the need to sacrifice code quality or testability.
MVVM Light Toolkit - The main purpose of the toolkit is to accelerate the creation and development of MVVM applications in WPF, Silverlight, Windows Store (RT) and for Windows Phone
Catel - Catel is an application development platform with the focus on MVVM (WPF, Silverlight, Windows Phone and WinRT) and MVC (ASP.NET MVC). The core of Catel contains an IoC container, models, validation, memento, message mediator, argument checking, etc.
UpdateControls - Update Controls does not require that you implement INotifyPropertyChanged or declare a DependencyProperty. It connects controls directly to CLR properties. This makes it perfect for the Model/View/ViewModel pattern.
ReactiveUI - An MVVM framework for .NET that integrates the Reactive Extensions (Rx) framework, enabling developers to build elegant, testable applications using WPF, Windows Store Apps, WP8 or Xamarin.
Okra App Framework - An app centric MVVM framework for Windows 8.1 built with dependency injection in mind, including a full set of Visual Studio MVVM templates.
Prism - A cross-platform desktop and mobile MVVM development framework.
WPF Application Framework (WAF) - A lightweight Framework that helps you to create well structured WPF Applications. It supports you in applying a Layered Architecture and the Model-View-ViewModel pattern.
MVVMCross - Cross-platform mvvm mobile development framework for WPF, Silverlight for WP7 and WP8, Mono for Android, MonoTouch for iOS, Windows Universal projects (WPA8.1 and Windows 8.1 Store apps). Makes extensive use of Portable Class Libraries (PCL) to provide maintainable cross platform C# native applications.
Stylet - Minimal MVVM framework inspired by Caliburn Micro, with good documentation, high test coverage, and its own IoC container
Gemini - IDE framework similar in concept to the Visual Studio Shell. Built on WPF, AvalonDock, and Caliburn Micro.
TinyMapper - A tiny and quick object mapper for .Net.
ExpressMapper - A lightweight, lighting fast .Net mapper to map one type of object(s) to another in automated and easy way. ExpressMapper relies completely on the expression trees.
Office
ExcelDna - ExcelDna makes it easier to create and deploy Excel Add-Ins using C#, F# or VB .NET
ClosedXML - ClosedXML makes it easier for developers to create Excel 2007/2010 files
AsyncPoco - A long-"awaited" fully asynchronous PetaPoco fork
NPoco - Simple microORM that maps the results of a query onto a POCO object. Based on Schotime's branch of PetaPoco
Limebean - Hybrid ORM which uses SQL fragments and doesn't require Model Classes.
LLBLGen Pro - Entity Modeling solution for Entity Framework, NHibernate, Linq to SQL and its own ORM framework: LLBLGen Pro Runtime Framework. [$][Free Lite version]
Insight.Database - Insight.Database is a fast, lightweight, micro-orm for .NET
DbExtensions - Data-access framework with a strong focus on query composition, granularity and code aesthetics.
ITextSharp - iText is a PDF library that allows you to CREATE, ADAPT, INSPECT and MAINTAIN documents in the Portable Document Format (PDF)[$][Free for OSS]
Profiler
MiniProfiler - A simple but effective mini-profiler for ASP.NET websites
Glimpse - The open source diagnostics platform for the web
Protocols
SSH.NET - A Secure Shell (SSH) library for .NET, optimized for parallelism. Provides SSH commands, SFTP/SCP uploads and downloads, and SOCKS4/SOCKS5/HTTP proxies.
Push Notifications
PushSharp - A server-side library for sending Push Notifications to iOS, OSX, Android, Chrome, Windows Phone, Windows 8, Blackberry, and Amazon devices.
Queue
NServiceBus - The most popular service bus for .NET
Hangfire - Incredibly easy way to perform fire-and-forget, delayed and recurring tasks inside ASP.NET applications
RabbitMQ.NET - Implementation of an AMQP client library for C#, and a binding exposing AMQP services via WCF
Rx.NET - The Reactive Extensions (Rx) is a library for composing asynchronous and event-based programs using observable sequences and LINQ-style query operators
Dynamic Data - Reactive Extensions (Rx) for collections
Scheduling
FluentScheduler - Task scheduler with fluent interface that runs automated jobs from your application
NCrontab - Class library for parsing & formatting crontab expressions as well as calculating occurrences of time based on a crontab schedule
SolrExpress) - A simple and lightweight query .NET library for Solr, in a controlled, buildable and fail fast way
Lucene.net - Lucene.Net is a port of the Lucene search engine library, written in C# and targeted at .NET runtime users
Serialization
Protobuf.NET - Protocol buffers is the name of the binary serialization format used by Google for much of their data communications
Json.NET - Popular high-performance JSON framework for .NET
ServiceStack.Text - JSON, JSV and CSV Text Serializers used in servicestack.net
Msgpack-Cli - MessagePack implementation for Common Language Infrastructure
Jil - Fast .NET JSON Serializer, built on Sigil (used by StackOverflow)
ProtoBuf - Generate C# code for protocol buffer serialization from a .proto specification.
F# Data - F# type providers for accessing XML, JSON, CSV and HTML files (based on sample documents) and for accessing WorldBank data
Bond - cross-platform framework for working with schematized data. It supports cross-language de/serialization and powerful generic mechanisms for efficiently manipulating data.
Wire - A high performance polymorphic serializer for the .NET framework.
FileHelpers - free and easy to use .NET library to import or export data from fixed length or delimited records in files, strings or streams.
FsPickler - A fast multi-format message serializer for .NET
Migrant - Fast and flexible serialization framework usable on undecorated classes.
State machines
Stateless - Create state machines and lightweight state machine-based workflows directly in .NET code
Automatonymous - A state machine library for .Net - allows you to write fluent style state machines
Appccelerate - State Machine - A powerful state machine library - configured with a fluent api and synchrounous and asynchronous state transition support
Static Site Generators
FsBlog - Blog aware, static site generation using F#
Pretzel - A site generation tool (and then some) for .NET platforms
Sandra.Snow - Jekyll inspired static site generation for .NET
Wyam - A simple to use, highly modular, and extremely configurable static content generator
xUnit - xUnit.net is a free, open source, community-focused unit testing tool for the .NET Framework
Canopy - Canopy is a free, open source F# web automation and testing framework
Tools
Fiddler - The free web debugging proxy for any browser, system or platform
Open Live Writer - Blog writer which integrated with WordPress, Blogger, et. al. Open Live Writer makes it easy to write, preview, and post to your blog. For more information see http://openlivewriter.org .
CodeHub - CodeHub is the best way to browse and maintain your GitHub repositories on any iOS device!
ShareX - ShareX is a free and open source program that lets you capture or record any area of your screen and share it with a single press of a key. It also allows uploading images, text or other types of files to over 80 supported destinations you can choose from.
Myrtille - A native HTML4/5 Remote Desktop Protocol client, HTTP gateway written in C# for .NET
Nuget Package Manager - NuGet is the package manager for the Microsoft development platform including .NET
SideWaffle - A collection of Item Templates for Visual Studio 2012/2013 that makes any web developer's life much easier
Resharper - Developer Productivity Tool for Visual Studio [$]
Refactoring Essentials - Open source extension for C# and VB.NET refactorings, including code best practice analyzers.
CodeContracts - Source code for the CodeContracts tools for .NET
Git Diff Margin - Displays live Git changes of the currently edited file on Visual Studio margin and scroll bar
Productivity Power Tools - A set of extensions to Visual Studio Professional (and above) which improves developer productivity.
VSColorOutput - Color highlighting for Build, Find and Debug output windows. Custom match patterns and colors can be added.
Developer Assistant - Developer Assistant for Visual Studio enables developers to find and reuse over 21 million code snippets and code sample projects from within the Visual Studio IDE.
Web Browsers
CefSharp - HTML5, CSS3 and JS web browser powered by Chromium, for WinForms and WPF
Web Frameworks
ASP.NET MVC - ASP.NET is a free web framework for building great web sites and applications
FubuMVC - A front-controller style MVC framework for .NET
NancyFx - Lightweight, low-ceremony, framework for building HTTP based services on .Net and Mono
Suave.IO - Framework/library/web server that makes you cry tears of joy after finishing your project ahead-of-time when you look at the beautiful code you've written in [F#].
Web Servers
Kestrel - A web server for ASP.NET Core based on libuv
EmbedIO - Web server built on Mono and cross-platform
XSP - Mono's ASP.NET hosting server. This module includes an Apache Module, a FastCGI module that can be hooked to other web servers as well as a standalone server used for testing (similar to Microsoft's Cassini)
WebSocket
Fleck - Fleck is a WebSocket server implementation in C#. Branched from the Nugget project
SignalR - Library for ASP.NET developers that makes it incredibly simple to add real-time web functionality to your applications
SuperSocket - SuperSocket is a light weight extensible socket application framework
Websocket-Sharp - A C# implementation of the WebSocket protocol client and server
WebSocket4NET - WebSocket client for .NET 2.0+, Xamarin, Mono, Silverlight, Windows Phone, & WinRT
Crossertech - Provides a great set of tools for you to build real-time applications on the Microsoft.NET plattform and much more. [$]
Awesome .NET Performance - A curated list of awesome .NET Performance books, courses, trainings, conference talks, blogs and most inspiring open source contributors.
awesome-ddd - A curated list of Domain-Driven Design (DDD), Command Query Responsibility Segregation (CQRS), Event Sourcing, and Event Storming resources
Awesome Xamarin - A collection of interesting libraries/tools for Xamarin mobile projects.
Resources
Weekly C# Digest - Weekly email newsletter with manually curated top 5 links from the .NET community.