Classy is an online application that takes a URL and parses it into a very useful list of all the CSS classes and IDs on the page. It breaks the classes down to show what HTML elements they're tied to, and does the same with each ID. Besides using the website version, you can also add Classy to your browser toolbar and run it on any web page you happen across.

Classy isn't a replacement for tools like FireBug or WebInspector, but it's a highly useful supplemental tool that gives you a quick overview of a web page's CSS structure that you can't easily get with those tools.