Since I'm only pretending to do web development myself, I'm not qualified to say, but Firebug is an awesome tool. If you're creating web apps of any kind, you should check out this free tool.

I'm not even going to try to do it justice by attempting to describe it to you. I'd fail miserably... the tool does too much for a blog posting. From it's HTML inspectors to CSS, Dom viewer, wire debugger, etc, it's an invaluable resource.

My favorite way to use it is with two monitors. The first monitor runs my browser window. The second runs Firebug (Tools/Firebug/Open Firebug in a New Window). You can also run it at the bottom of your browser screen, but I find it much less useful there. I simply can't see enough of Firebug when it's jammed in at the bottom. It's better than no Firebug at all, but once you've tried it dual monitor, you may have trouble returning to a single screen. You've been warned. :)

It's one potential drawback (depending on your shop)... it only runs in Firefox, but since FF is free and cross platform, Firebug should still be a valuable tool for the IE only developers.

Here are the links:

Firebug Homepage

FireFox add ons page

The first link is easier if you want to look at features. The second is easier if you just want to download and install.