FIREFOX:
Webdeveloper is a very useful Firefox extension for webdesigners and skinners. Too many cool features to list here.
http://www.chrispederick.com/work/firefox/webdeveloper/
IE:
If you have ever wanted to install (multiple) old versions of IE for use in webdesigning then here's a solution.
http://skyzyx.com/downloads/
Any other tools that we should know about?
-=DKC=-
I agree, the webdeveloper plug-in for Firefox is definitely awesome. It's probably the tool I use the most. I especially like to be to edit the CSS of a page and instantly see the results.
I've found this Color Picker (http://www.iconico.com/colorpic/index.aspx) to be handy.