The YUI Library debuted at Yahoo in 2005 and was open-sourced under a BSD licence in February 2006. Since its release, YUI has been integrated into most of Yahoo!'s properties, including the Yahoo! home page, Yahoo! Mail, and Flickr. On the YUIBlog, we've profiled companies like LinkedIn, Mint, SugarCRM and PowerReviews who have adopted YUI as their frontend platform. And all of that is just the tip of the iceberg. Check out the "In the Wild" column on YUIBlog for many more examples of YUI being used across the web.
One of the hottest social-networking sites in the world is built using YUI. Hear why from LinkedIn's developers.
The YUI Library is compatible with and extended by many other frontend libraries. Dustin Diaz's DED|Chain, for example, takes the YUI foundation and sugars it up with a chaining syntax. Jack Slocum's Ext JS, which originally began as a set of YUI extensions, can employ the YUI core and thereby adds all of the great Ext components to the YUI ecosystem. Here are a few sites to explore as you orient yourself to all of the resources available in the YUI developer community:
All YUI 2.x users should review the YUI 2.8.2 security bulletin, which discusses a vulnerability present in YUI 2.4.0-2.8.1.