I would recommend looking at Mootools (http://mootools.net) if you're not already familiar with it. It's my personal favorite JS library, followed by YUI (http://developer.yahoo.com/yui/). Both are some of the newest libraries on the block. YUI is probably the most respected and well-thought-out library out there. I personally prefer Mootools over it though, since I'm a fan of its syntax and greater emphasis on code brevity.
I just starting playing with JQuery and it looks pretty awesome.