Disclaimer The opinions expressed herein are my own personal opinions and do not represent my employer's view in any way.
I saw an article about Qooxdoo (pronounced like "cooks-doo") on the Ajaxian website this evening. A JavaScript Framework that has apparently been in develoment since 2005.
The Qooxdoo framework is open source, and has some RPC hooks for Java,Perl, and PHP from what I saw. I am posting this as a note to go look into that a bit more later on. They make a point that this is not a Javascript Library - like jQuery or prototype it is a Framework.
There is a PlayGround that the article mentioned - so you can play around with the Javascript API.
More interesting software to look at.