CSS Lint : CSS Lint points out problems with your CSS code. It does basic syntax checking as well as applying a set of rules to the code that look for problemati...
PurgeCSS : PurgeCSS is a tool to remove unused CSS. It can be part of your development workflow.
Selectivizr : CSS3 pseudo-class and attribute selectors for IE 6-8
320 and up : ‘320 and Up’ is a lightweight, easy to use and content first responsive web design boilerplate.
UnCSS : UnCSS is a tool that removes unused CSS from your stylesheets. It works across multiple files and supports Javascript-injected CSS.
PostCSS plugin to parse CSS and add vendor prefixes to CSS rules using values from Can I Use.