On Monday Yahoo! have posted an amazing "Yahoo! User Interface Library", an AJAX-style UI library for development of dynamic web user interfaces. It was released under BSD license which means that it's free for all developers to implement and deploy their own solutions using this library. Very cool! Check out some samples. So far the Sliders are my favorite!  Every control and feature provided with API documentation and there is a companion blog with a lot of information.

Another library that's came out of is Yahoo! Design Pattern Library, the collection of most commonly used interface design features. Very well structured and documented, nice reading for every web-developer!