DoJo Toolkit

http://dojotoolkit.org/

from the web site:

What is Dojo?

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. Dojo lets you prototype interactive widgets quickly, animate transitions, and build Ajax requests with the most powerful and easiest to use abstractions available. These capabilities are built on top of a lightweight packaging system, so you never have to figure out which order to request script files in again. Dojo's package system and optional build tools help you develop quickly and optimize transparently.