Yahoo! releases User Interface library14 Feb 2006
Yahoo! has released it's User Interface library and Design Pattern library under an Open-Source license(BSD). I believe, Yahoo! has been using these internally but now they made it public.
I have had a chance to look at UI library which consists of utility classes and widgets(ui controls). Things like Drag-Drop, Custom Event, AJAX handling etc are so cool to use; few lines of code and you are done. Similarly, widgets like Tree, Slider, Calendar are also very easy to use...
If you are into eLearning domain, you would love this cross-browser library to make eLearning interaction. Which require drag-drop, animation etc.
If you are into RIA development and looking for AJAX as an option, you should look at the Connection Manager along with other classes.
Update: You can also look at Y! UI Blog.