The Yahoo Developer Network
I was hunting for for an JavaScript auto-completion script when I stumbled across this:
http://developer.yahoo.com/yui/
Seems to be in fashion for large web companies to go open-source these days. Yahoo has provided its UI library for download - open-source under a a BSD compatible license.
Infact, there seems to be quite a lot on the rest of the site as well - the “Yahoo Developer Network“.