Firebug - Mozilla

Firebug Firebug makes it simple to find HTML elements buried deep in the page. Once you've found what you're looking for, Firebug gives you a wealth of information, and lets you edit the HTML live. Firefox has a "View Source" window, but it doesn't show you what the HTML source really looks like once it has been transformed by JavaScript. Firebug's HTML tab shows you what the HTML looks like right now.

Firebug's CSS tabs tell you everything you need to know about the styles in your web pages, and if you don't like what it's telling you, you can make changes and see them take effect instantly.

For free download, documentation or update visit: http://getfirebug.com/