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.
Deadweight is RCov for CSS, kind of. Given a set of stylesheets and a set of URLs, it determines which selectors are actually used and reports which can be "safely" deleted.
Enter the URL of any CSS file to view and modify its color spectrum
organizes and standardizes your CSS - selectors, sub-selectors and properties - in a structure that makes it easier to define page areas and see how they relate to each other.