SubscribeIf you’re a web developer and you don’t have Firebug, go get it: http://getfirebug.com/. It’s got all kinds of good stuff for you if you work with html, css, javascript, etc.
I’d used earlier versions, but it wasn’t until recently that I started looking into the features 1.0 has. Back when I started using it it was “just” a javascript debugger and dom inspector. Now it shows http headers, css, can log from javascript (much nicer than alerts). It tells you how long each page, image, and script took to load. Html and css can be edited right in the browser and your changes show up in the page as you type them
Just go to their page to see everything it can do.