csscss will parse any CSS files you give it and let you know which rulesets have duplicated declarations.
CSS Coverage is an extension for Firebug which allows you to scan multiple pages of your site to see which CSS rules are actually used in your site. This extension requires Firebug.
cssess is a JS tool that helps find extra CSS rules on your site. It is designed to be used as a bookmarklet.
A SASS mixin for writing media queries with a clean and natural syntax
Helium - javascript tool to scan your site and show unused CSS
LiveReload guard allows to automatically reload your browser when 'view' files are modified.
CSS spinner for modern browsers, built with Sass