CSS プリプロセッサ

Sass (サス) - Syntactically Awesome Style Sheets
SCSS (エスシーエスエス) - Sassy CSS
Sass: Syntactically Awesome Style Sheets

Less (レス) - The Dynamic Stylesheet Language
Getting started | Less.js

Stylus (スタイラス) - Expressive, dynamic, robust CSS
Expressive, dynamic, robust CSS — expressive, robust, feature-rich CSS preprocessor

PostCSS (ポストシーエスエス) - A tool for transforming CSS with JavaScript
PostCSS - a tool for transforming CSS with JavaScript

CSSプリプロセッサのまとめ　主にSsss(scss)とPostCSS - Qiita
CSSプリプロセッサ（CSSメタ言語）のお勉強 | ホワイトボードオフィシャルブログ