I’ve been messing about with Firebug, a web development extension for Firefox. The site itself does a much better job of explaining FB than I ever could, so I would direct you over there for definitive information.
At base, FB exposes everything about the webpages you view, from HTML to CSS to JavaScript to the download times for each portion of your page. It also allows you to change the CSS values for any given DOM element, meaning that you can preview HTML changes in real time. It’s really slick, I have to say. If you have any web design inklings, I’d pretty much say you have to check out Firebug.
Firebug: A Web Development Must-Have
My new favorite feature: When you inspect an element in the left pane, and you see the CSS in the right pane, you can hover over the CSS that references a background image and see the freaking image and its dimensions in a tooltip. Wow!