Gmail
From userscripts.org, a Greasemonkey script to organize your Gmail labels into folders. Folders4Gmail (Userscripts.org)
Get extra, customizable keyboard shortcuts and macros with Greasemonkey and Gmail with Modified Gmail Macros v2.0 (userscripts.org)
Web Development
60+ Useful Adobe Air Applications You Should Know. (hongkiat.com)
20 Excellent AJAX Effects You Should Know (NETTUTS)
"Why use flash for web menus when javascript has enough power to achieve similar effects".13 Awesome Javascript Animated Flash Like Menus (devSnippets)
How to Create a Stunning and Smooth Popup Using jQuery. Check out the demo. It's smooth! (yensdesign.com)
21 Most Useful Free Resources for Designers and Web Developers (smashing apps)
Javascript Calendars for Lazy Webmasters (javascript-examples.com)
typeface.js allows you to embed custom fonts in your web pages so you don't have to render text to images. (typeface.neocracy.org)
Firefox
Five Firefox 3.0 a new look with 20 Visually Beautiful Firefox 3.0 Themes (1stwebdesigner.com)
CSS
So glad to see webmonkey back! Browser-Specific CSS Hacks (webmonkey)
Innovative and Experimental CSS Examples and Techniques (Six Revisions)
Using Reset Stylesheets, a primer from webmonkey. (webmonkey.com)
Progressive Enhancement with CSS. "There are many ways to integrate progressive enhancement into your work using Cascading Style Sheets (CSS), and this article will cover a few of the biggies and get you thinking about other ways to progressively enhance your sites. " (A List Apart)
Design
The Twenty Most Earth-Shattering Tutorials on NETTUTS (NETTUTS)
GIMP
1000+ Free High Resolution GIMP Brushes (noupe.com)