Dojo: AJAX in Sun style
Dojo is the Open Source JavaScript toolkit that helps you build serious applications in less time. It fills in the gaps where JavaScript and browsers don't go quite far enough, and gives you powerful, portable, lightweight, and tested tools for constructing dynamic interfaces.
This project has Sun behind itself: on Downloadblog there is a post dedicated to a Videocast of Sun Developers about Dojo and Java Studio Creator.
The “packages” organization of Dojo is very similar to Java Library: much coherent and clean.
It’ better than Prototype? I can’t try Dojo right now. ;)