Jack Slocum's wonderful Ext library, which was originally based upon the Yahoo! User Interface (YUI) components, has now gone live with a universal version that supports jQuery and Prototype/Scriptaculous as well. You can actually customize your download package based upon your JS library of choice and the widgets that you'd like available.
Kudos to Jack for putting out such a professional package - lots of good stuff to play with for creating some serious desktop apps!
Check it all out and download at http://extjs.com/