Yahoo!
I stumbled into the design patterns piece of it on del.icio.us Monday, but during my periodic look at mezzoblue, the details became more clear. Not only has Yahoo made available to the public a set of design patterns, but a UI library and other tools.
Rather than duplicating details here, I'll pass you to Chad Dickerson's more detailed summary. (Thanks to flashpoint for the ref to Chad's words on the user interface library.)