The YUI is the Yahoo User Interface library? It's a:
The whole lot is licensed with FreeBSD and yours to take and use for whatever you please.
The Yahoo! Developer Network (or YDN for short) has all the documentation and other information you'll need from Yahoo! on their developer resources. This includes APIs, a pattern library and news about future development. Visit the YDN.
The Yahoo! User Interface blog has the latest information on all things YUI and scripting. Visit the YUI Blog.