dojo Javascript Toolkit

From dojotoolkit.org:

dojo core

Ajax, events, packaging, CSS-based querying, animations, JSON, language utilities, and a lot more. All at 24K (gzipped).

dojo dijit

Skinnable, template-driven widgets with accessibility and localization built right in—the way you want it. From accordions to tabs, we have you covered.

dojoX

Inventive & innovative code and widgets. Visualize your data with grids and charts. Take your apps offline. Cross-browser vector drawing. And a lot more.