Web Devloper Extension.
I'm deep in the world of CSS and I've been going through 2 of the best references on the topic. They are Web Design in a Nutshell and Bulletproof Web Design. Both are immensely informative, succinct and astoundingly useful. Both of these authoritative tomes reccomend the Web Developer extension for Firefox, as an aid to web development. It's soooo awesome - if you web designers out there haven't tried it yet... Well, you just suck.