The UI Library Utilities facilitate the implementation of rich client-side features by enhancing and normalizing the developer’s interface to important elements of the browser infrastructure (such as events, in-page HTTP requests and the DOM).
The Yahoo UI Library Controls produce visual, interactive user interface elements on the page with just a few lines of code and an included CSS file.
All the components in the Yahoo! User Interface Library have been released as open source under a BSD license and are free for all uses.
For more info reach here Yahoo! UI Library