What follows are some of my favorite front-end development resources (aside from Firebug, and the jQuery library!)
1. HTML5 Boilerplate. Prepare for the coming of HTML5. Use this mini-framework. Heck, use this for XHTML. Contains lots of goodies, no matter your flavor of HTML.
2. ie7.js. A JavaScript library to make MSIE behave like a standards-compliant browser. I have yet to test it so, stay tuned, or give me your feedback. If it does what it promises, could be one of the best contributions to the web design community since a non-IE browser.
3. Google Font API. A must-have if you want to get away from the usual 5 or 6 web-safe fonts.
4. Easy favicon generator. Not only can you import your own image (select the resize option), you can touch it up pixel by pixel once imported.
5. My new favorite: PIE. The brilliant minds at css3pie.com have conceived of a method to quickly and more cleanly add css3 effects to a page rendered in an Internet Explorer. What wins me over: rounded corners.