View Source Developer Tool. Wicked!
View Rendered Source Chart -- This is one of the coolest web developer tools I've ever seen! It is a Firefox plugin which color codes the source text in beautiful tables making it very easy to steal other peoples code debug your own code.
Check out the page, see the examples and I know you'll be hooked. Just don't look at my code, it's a mess; 9 years of web site cruft, things build up.
Mac OS X users, there's a stand-alone app which does something similar but a little more powerful called Xyle Scope, for those more complicated debugging sessions. Courtesy of Sean Sitter