I saw this, and it sounds really cool. A couple of other good links I found in the discussion:
http://prototype.conio.net/
http://script.aculo.us/
The first sounds like a very robust ajax library :-)
from a slashdot news :)
"Today Yahoo! released the Yahoo! User Interface Library. This library is comprised of a number of dynamic HTML utilities and controls for building rich web UIs and Ajax applications. They are made available under an open-source license. In addition, Yahoo! released the Yahoo! Design Pattern Library. This collection of design patterns for Web interaction is intended to provide Web designers prescriptive guidance to help solve common design problems on the Web. Both are free in both senses of the word."
http://developer.yahoo.net/yui/
http://developer.yahoo.net/ypatterns/