cssess is a JS tool that helps find extra CSS rules on your site. It is designed to be used as a bookmarklet.
Helium is a tool for discovering unused CSS across many pages on a web site. The tool is javascript-based and runs from the browser.
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.