script.aculo.us
We have been using script.aculo.us libraries effectively in most of our web applications. It is a set of JavaScript libraries that bring in some value add in terms of providing on the fly rich UI stuff. Moreover it is a neat implementation that hides complex JavaScripts and loads them behind the scenes. This makes your code distinct from the libraries and makes it clean. I would highly recommend using script.aculo.us if you want to develop rich UI sites really quickly.
