I've been using the scriptaculous libraries for a while now and they recently released a version which incorporates the latest Prototype.js library

Prototype (http://prototypejs.org/) is a highly complex framework for JavaScript developers.  While i don't pretend to know anything about mostly, i know that the Scriptaculous libraies use them extensively to provide it's animations in a consistent format

http://script.aculo.us gives a very easy-to-use set of animation effects for use on web pages.  By using the prototype library, Thomas Fuchs has been able to create all sorts of nice AJAX effects for us to use free of charge.  This integrates natively with Ruby on Rails and a lot of his code samples are Ruby-Oriented, especially all the drag and drop functionality and how it connects to the server behind-the-scenes

It integrates nicely with .net as well, even when using the AJAX.NET framework :¬)

Check out the demos page >> http://wiki.script.aculo.us/scriptaculous/show/CombinationEffectsDemo