Drag and drop library, also things like scale and resize images in the browser. Probably the most sophisticated of the lot. Simple, efficient.

Animation framework, drag and drop, Ajax controls, DOM utilities, and unit testing.

UI library based on prototype.

Layout managers, empty application program templates is more like it.

A UI for jquery.

UI widget framework.

A collection of UI widgets called... wait for it... "interface".

The Ext layout manager shimmed with mootools. Most of it works. Impressive.

Comprehensive set of widgets and one of the most complex and comprehensive demos ever built: portalfin.com