Kudos to Yahoo! for releasing their javascript library under BSD. The Yahoo! UI Library includes js for animation, xmlhttp connection handling, DOM, drag and drop, events, and complex ui components such as cals and trees.

I recently started to use the Dojo toolkit in an application that we've been building at Boingo Wireless, and it has been great. But YUI is going rocket ahead of Dojo and the others, for the obvious reason that Yahoo has one of the best web services endpoints out there. The fact that they offered JSON so early in the game shows how far they'll stick their necks out to stay ahead of the competition.

Also worth a look is the Yahoo! Design Pattern Library. It contains some great articles explaining use cases, solutions, and rationales for many conventional (or soon-to-be conventional) web application elements.

Spotter (probably from down the hall): Jeremy Zawodny.