A cool Date Picker?
A Draggable element on the client side?
Cool animation effects?
A Sweet Model dialog?
It just makes client side programming so nice :)
If you have experience with JQuery, Prototype, script.aculo.us, YUI or anything else that Google Helps, put together a demo, let me know and show off some stuff!
If you want to try to create something first: you can check out http://code.google.co... (http://code.google.com/apis/ajaxlibs).