YUI 3 is Yahoo!’s next-generation JavaScript and CSS library; it powers the new Yahoo! homepage and incorporates what they have learned in five years of dedicated library.  development.

The YUI libraries are open-source, freely available, and used widely around the Internet for Web site tasks including animation, drag-and-drop, fetching data from various types of sources, and responding to events–chores that are more complicated but that often are useful as the Web moves from static Web pages toward interactive applications.

Read More: Yahoo Blog, Yui