Sass is an extension of CSS that adds power and elegance to the basic language. It allows you to use variables, nested rules, mixins, inline imports, and more, all with a fully CSS-compatible syntax. Sass helps keep large stylesheets well-organized, and get small stylesheets up and running quickly.
Sass has two syntaxes. The new main syntax (as of Sass 3) is known as "SCSS" (for "Sassy CSS"), and is a superset of CSS's syntax. This means that every valid CSS stylesheet is valid SCSS as well. SCSS files use the extension .scss.
The second, older syntax is known as the indented syntax (or just "Sass"). Inspired by Haml's terseness, it's intended for people who prefer conciseness over similarity to CSS. Instead of brackets and semicolons, it uses the indentation of lines to specify blocks. Although no longer the primary syntax, the indented syntax will continue to be supported. Files in the indented syntax use the extension .sass.
Stack Overflow - What's the difference between SCSS and Sass?
Frameworks
Bootstrap 4140k 77k - Bootstrap version 4, the most popular HTML, CSS, and JS framework for developing responsive, mobile first projects on the web.
Bootstrap-sass13k 4k - Official Sass port of Bootstrap 2 and 3.
Foundation for Sites8k 3k - 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.
Scooter - SCSS framework built to provide base styles, CSS components, and rapid static prototyping for Dropbox.
Sierra - Micro SCSS library to help you build websites, without all the arbitrary selectors.
Kickoff - Kickoff is a lightweight front-end framework for creating scalable, performant and responsive sites.
Hocus-Pocus - Universal and lightweight stylesheet starter kit that focuses on base html elements and typography.
Materialize - Modern responsive front-end framework based on Material Design.
Bulma25k 2k - Modern CSS framework based on Flexbox.
iotaCSS - Open source Sass-based OOCSS framework built for scale.
include-media - Simple, elegant and maintainable media queries.
Sass MediaQueries - Collection of useful media queries mixins for Sass (including iOS devices, TVs and more).
Sass MQ1k 100 - Sass mixin that helps you compose media queries in an elegant way.
Breakpoint2k 156 - Breakpoint makes writing media queries in Sass super simple.
mq-scss40 3 - An extreamly powerful but easy to use Sass media query mixin.
Color
scss-blend-modes804 134 - Using standard color blending functions in Sass.
brand-colors - 1100+ collection of popular brand colors available in Sass, Less, Stylus and CSS.
Open color3k 138 - Open color is a color scheme for UI design. Available in CSS, SCSS, LESS, Stylus, Adobe library, Photoshop/Illustrator swatches and Sketch palette.
sass-planifolia3 1 - Advanced color manipulation and contrast calculation in vanilla Sass.
Typography
Sassline - Set text on the web to a baseline grid with Sass & rems using a responsive modular-scale.
Typi648 33 - Sass mixin to make responsive typography easy.
Sassy-Gridlover199 16 - Super easy to use Sass mixins to establish a typographic system with modular scale and vertical rhythm. Based on the Gridlover app.
Shevy - Typography made easy. A vertical rhythm library.
Hover - Collection of CSS3 powered hover animated effects to be applied to links, buttons, logos, SVG, featured images and so on. Available in CSS, Sass, and LESS.
Miscellaneous
Bourbon - Simple and lightweight mixin library for Sass.
Family.scss - 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-scss1k 350 - Sass/Compass version of Normalize.css, a collection of HTML element and attribute rulesets to normalize styles across all browsers.
Buttons5k 1k - CSS button library built using Sass and Compass.
Modular Scale794 91 - Modular scale calculator built into your Sass.
Scut795 79 - Collection of Sass utilities to ease and improve the implementations of common style-code patterns.
Sass flexbox mixin1k 285 - Set of mixins for those who want to mess around with flexbox using the native support of current browsers.
Angled Edges714 50 - Sass mixin for creating angled edges on sections by dynamically encoding SVGs.
retina.js4k 759 - JavaScript, SCSS, Sass, Less, and Stylus helpers for rendering high-resolution image variants.
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.
Scout-App - Process your Sass and SCSS files into CSS without needing any knowledge of the command line.
sass-rails775 363 - Ruby on Rails stylesheet engine for Sass.
scss-lint2k 496 - Configurable tool for writing clean and consistent SCSS. (deprecated)
OctoLinker1k 123 - Navigate through *.scss and *.sass files efficiently with the OctoLinker browser extension for GitHub.
stylelint - A mighty, modern CSS linter that helps you enforce consistent conventions and avoid errors in your stylesheets. Supports CSS-like syntaxes, including SCSS.
diamond - Dependency management built for Sass, Less, and CSS.
node-sass-magic-importer52 7 - Custom node-sass importer for selector specific imports, node importing, module importing, globbing support and importing files only once.
SharpScss8 4 - P/Invoke .NET wrapper around libsass to convert SCSS to CSS supporting NET2.0/NET3.5/NET4.x+ and CoreCLR platform.
sass-extract63 6 - Extract variables from scss files. Use scss to describe styles for use in javascript by extracting computed styles into js objects. Supports imports and advanced language features.