Firebug http://www.getfirebug.com/ is one of the coolest web development tools I have seen since Web Developer Extension for Mozilla.

From their site (they are too modest):

"Firebug integrates with Firefox to put a wealth of development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page. "

So far I have only used it for debugging JS so that is all I am going to comment on.

If you have ever debugged Java Script you know how much of a pain it can be, especially if you're JS is generated on the fly. Debug with IE? Forgidabodit !!!

• You can walk through a JavaScript and set break points and watches just like in Visual Studio.
• Quickly find JS errors.
• You can search for variables and strings right from your browser.
• You can view attached CSS and JS files, http headers, the referrer page, cookies

There is way, way, WAY too much to list here so just do yourself a favor and install it. You will thank me for it.