Dust-Me Selectors

Dust-Me Selectors is a Firefox extension (for v1.5 or later) that finds unused CSS selectors.

It extracts all the selectors from all the stylesheets on the page you're viewing, then analyzes that page to see which of those selectors are not used. The data is then stored so that when testing subsequent pages, selectors can be crossed off the list as they're encountered.

You can test pages individually, or spider an entire site, and you'll end up with a profile of which selectors are not used anywhere.

Core support

Supports local and remote stylesheets included using <link> elements, <?xml-stylesheet?> processing instructions, and @import statements. (Note: rules inside <style> blocks that are not @import statements, and inline style attributes, are not included.)
Supports <link> stylesheets inside IE Conditional Comments.
Check a single page, or an entire site.
Understands all CSS1 selectors, and most CSS2 and CSS3 selectors (courtesy of Dean Edwards' wonderful base2).
Caters for common CSS hacks — for example, "* html #foo" will be treated as "html #foo".