Stack Overflow - What's the difference between SCSS and Sass?
Frameworks
Bootstrap 4 - Bootstrap version 4, the most popular HTML, CSS, and JS framework for developing responsive, mobile first projects on the web.
Bootstrap 3 - Bootstrap version 3, the most popular HTML, CSS, and JS framework for developing responsive, mobile first projects on the web.
Foundation for Sites - The most advanced responsive front-end framework in the world. Quickly create prototypes and production code for sites that work on any kind of device.
Bourbon - A simple and lightweight mixin library for Sass.
Scooter - An SCSS framework built to provide base styles, CSS components, and rapid static prototyping for Dropbox.
Sierra - A micro SCSS library to help you build websites, without all the arbitrary selectors.
Libraries and Mixins
include-media - Simple, elegant and maintainable media queries.
Family.scss - A set of 26 smart Sass mixins which will help you to manage the style of :nth-child’ified elements, in an easy and classy way.
normalize-scss - Sass/Compass version of Normalize.css, a collection of HTML element and attribute rulesets to normalize styles across all browsers.
scss-blend-modes - Using standard color blending functions in Sass.
Buttons - A CSS button library built using Sass and Compass.
brand-colors - 1100+ collection of popular brand colors available in Sass, Less, Stylus and CSS.
Sass flexbox mixin - A set of mixins for those who want to mess around with flexbox using the native support of current browsers.
Sass MediaQueries - A collection of useful media queries mixins for Sass (including iOS devices, TVs and more).
Angled Edges - A Sass mixin for creating angled edges on sections by dynamically encoding SVGs.
Hover - A collection of CSS3 powered hover effects to be applied to links, buttons, logos, SVG, featured images and so on. Available in CSS, Sass, and LESS.
retina.js - JavaScript, SCSS, Sass, Less, and Stylus helpers for rendering high-resolution image variants.
Typi - A sass mixin to make responsive typography easy.
Open color - Open color is a color scheme for UI design. Available in CSS, SCSS, LESS, Stylus, Adobe library, Photoshop/Illustrator swatches and Sketch palette.
Sass Accoutrement - Accoutrement modules are Sass toolkits that work together to form the central configuration of a project. The tools can be used individually, or integrated for extra power.