A tip of the hat to Dori Smith for pointing out the Yahoo! UI Library (YUI): “The Yahoo! User Interface (YUI) Library is a set of utilities and controls, written in JavaScript, for building richly interactive web applications using techniques such as DOM scripting, DHTML and AJAX. The YUI Library also includes several core CSS resources. All components in the YUI Library have been released as open source under a BSD license and are free for all uses.”
Plenty of resources to help you with the YUI components such as a blog, a forum, documentation. Good stuff.