DOM
Tools and guides for working with the document object model (DOM)
Statsy DOM inspection bookmarklet
Statsy (http://www.phpied.com/statsy-more-data-points-for-markup-quality/) is a bookmarklet that does a quick DOM analysis of the page that you run it on. Tells you how much code you have inlined in your HTML, which I gather isn’t great for performance and can sometimes result in the dreaded FOUC.
Firebug
Firebug is a firefox plugin that allows you to inspect the DOM, change a page’s CSS and HTML on the fly, and much more. Includes a javascript console and all kinds of other stuff. If you’re a web developer, you absolutely need this plugin.