Firebug is probably one of the best tools web-developers.
I'm using it for some years now and I remember what it was like before it. It literally changed my life :). Everything is much easier and I can do more complex stuff, quicker.

Firebug is an extension for Firefox. It sits quietly until you decide to use it. It comes with:

• console: a good tool for javascript monitoring. Great for Ajax calls as well.
• inspector: get a hold of your html & css. This is probabily the best feature, and once you start using it you'll never be able to stop.
• net: lets you analyze your page, great for optimization
• script: a great tool for debuging your scripts

In addition, extend your firebug with YSlow a wonderful tool release by Yahoo that helps you optimize sites, a must have and must do.

Microsoft has introduced with IE8 has a tool that resembles Firebug all too well. In fact, one might say it's a clone, a not so well looking one.
However, Firebug has a few aces in its sleeve and Firefox + Firebug still kicks ass.

Get Firebug | Don't have Firefox yet?