Say you had a dynamically generated site that has been worked on by too many people, past and present, and you now have a collection of shared stylesheets that contain over
I've used Dust-Me Selectors before, which is a Firefox plugin. It's very easy to use and versatile as it maintains a combined list across a number of pages of which CSS values are used.
The downside is that I wasn't able to automate it to spider an entire site, so I ended up using it just on key pages/templates of my site. It is very useful nonetheless.
http://www.sitepoint.com/dustmeselectors/
https://addons.mozilla.org/en-US/firefox/addon/dust-me-selectors/
Contrary to the comment above Dust-Me Selectors 2.2 is compatible with Firefox 3.6 (I've just installed it).