Whoa. You know, it's one thing to give out free APIs for people to use in order for them to tie directly into your services. That's kind of what Yahoo! Maps, Flickr, and other APIs are for: You to write something damned cool, yet still hook you into Yahoo! services. This Yahoo! User Interface Library is something completely different and cool. From the website:
The Yahoo! User Interface Library is a set of utilities and controls, written in JavaScript, for building richly interactive web applications using techniques such as DOM scripting, HTML and AJAX. 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.You download it. You integrate it into your site. There's no hooks back to Yahoo!, it's all for you to use to make your website better. I like that a lot. That is very cool.