No mention of YUI yet? I have been using it for years and can't imagine using any other framework.

YUI can be somewhat low level, but YUI 2 has grown to provide many impressive components like layout manager, menus, data tables, rich text editors, tree views, tab views, and more. Not to mention one of my favorite parts of YUI, the CSS font and reset files.

YUI 3 is much more reminiscent of jQuery, but doesn't quite have the component list of its predecessor (YUI 2). The YUI community is very active and the people behind the project are very communicative.

Anyways, just thought I would throw in my vote for YUI.