script.aculo.us – web 2.0 javascript
If you develop web apps and you haven’t heard of this, go check it out. It makes it easy to add great functionality to your apps like drag-n-drop, visual effects and auto-completing text fields. There is another article that provides a way to use script.aculo.us in S5 slides….which you should also check out. 
http://labs.cavorite.com/presentacular/
For some demos of this technology, check this out: