jQuery Project
I’ve been a big fan of jQuery more or less since it came out, and I’m happy to see the launch of The jQuery Project. I’ve used jQueryUI a couple of times and find it fairly useful; I haven’t tried Sizzle yet but it looks great for situations where you’re really concerned about keeping file sizes low but need decent CSS selector support; and I wish QUnit had been around when I was writing a lot of client-side code.