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