CSSViewer - A simple CSS property viewer
CSS Next - Use tomorrow’s CSS syntax, today.
CSS Animation Cheat Sheet - Awesome CSS3 plugin for spiffy animations
Garden (Clojure) - Unlike the mini-languages that are other pre/post-processor options, Garden leverages the full power of the Clojure programming language for CSS.
Tailwind CSS - A utility-first CSS framework for rapidly building custom user interfaces.
Stylus - EXPRESSIVE, DYNAMIC, ROBUST CSS