Best libraries for JS
I would vote for JQuery.
The yahoo UI library is cool, but it is slow and sometimes too complicated. Extension JS is another library built on top of such libraries. And it almost makes JavaScript just like Java.
You can find it here. [http://www.extjs.com]
Read all about JQuery here. [http://www.jquery.com]
JQuery is brilliant in terms of it's simplicity, speed and ease of learning. You can ask just about any questions on JQuery here and we would try to answer your questions.
Ext JS is also being used extensively at our end. We should be able to help you in case of a need. All you need to do is do a post!