I realize I'm probably the last to get on the boat with this one, but I came across the Yahoo! UI Library
this morning and it's pretty cool stuff. It's an open-source set of UI components for creating rich web applications. Items I know I've had to write that are included:
They have a lot of other neat stuff, too, like menu controls, treeviews, animation libraries... it's pretty slick. I'm going to have to play around with this and see how it works. It looks promising.