There are a number of helpful in-brwoser web development tools out there. I have used Firebug and the Web Developer Toolbar for quite some time. Recently, I have added some additional features like YSlow and Live HTTP Headers which have proved to be helpful. I know there are a number of other tools out there and Smashing Magazine recently posted a list of 15 great tools.

Check them out: http://www.smashingmagazine.com/2008/11/18/15-helpful-in-browser-web-development-tools/