I've been browsing through the Javascript widget library provided freely by Yahoo!, there are some nice examples here showing off some of the features on offer:

Yahoo! UI Library (YUI)

A few years back this would have been called a DHTML library, but somehow that term has fallen out of fashion despite the underlying technologies being the same, that's the marketing of technology for you ;-)

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Updated 2021 : note that the above post is out of date as it was originally published in 2007, but is left here for archival purposes. Unfortunately as of 2014, the YUI project is no longer supported by Yahoo! - https://yahooeng.tumblr.com/post/96098168666/important-announcement-regarding-yui