- ngudmuns
After working at UTO for a bit, I was introduced to a firefox add-on called firebug (thanks Chris!). It's totally awesome. I am ashamed to admit I actually used to code for IE and then fix bugs in firefox (coding for an internal app, where all the users were IE-based). Of course, that's completely backwards! Especially with the momentum Firefox is gaining, hopefully soon coding for Mozilla first will reach the largest audience.
 After I found out about firebug, I couldn't stand working with IE. It makes css debugging a BREEZE, instead of a horrible experience filled with lots of head-banging-against-wall moments :) I then proceeded to show it to many people I work with and others that are interested in web design. Some said "well, yah, that's an excellent extension." Even more said "wow, I can't believe I haven't seen that before."  You can download it here.
 Give it a try, I'm 100% sure you won't be disappointed. Make sure you check out the "Inspect" feature.
Firebug is an invaluable tool for debugging Javascript (especially AJAX-type stuff), as well. Also, be sure to check out YSlow from Yahoo. It help you locate performance issues with your page loads and integrates seamlessly with Firebug.
Y-slow is great! I also really like the web developer toolbar. I have used that ruler so many times! :)
Firebug is a neccesity for troubleshooting CSS and JS probs. I love inspecting the DOM, modifying the style right there, and seeing how it affects the site instantaneously. Would love it if someone came out with a good Eclipse plugin for it, so the style sheets could be modified and saved through Firebug. I see there is something called Fireclipse, but it doesn't look available yet.