Browser Development Tools

Firefox

• Venkman - JavaScript debugger. Bit of a head scratcher at first-- you need to navigate to your source, set a breakpoint, and then load the page in FireFox. Execution will stop when the breakpoint is reached (note this means you also can't look up other webpages will debugging)
  
• DOM Inspector - allows you to navigate the DOM of a page loaded in Firefox. Just press Ctrl-Shift-I to load the Inspector
• Developer Toolbar - really handy tool bar that outlines elements and shows all sorts of other handy info; also allows editing of source and CSS in real time
  

Interner Explorer

• Script debugger - IE6 only - very buggy debugger. Crashes often. But it's the only way to see into IE's crazy mind.
  
• Fiddler and Fiddler 2 - plug-in that intercepts communications between IE and sites and logs it. You can inspect all sorts of data and edit it. I found Fiddler to be more stable, but Fiddler 2 has some limited support for HTTPS.
• IE Developer Toolbar - IE6 only - kind of like the DOM inspector for Firefox but in a toolbar