They're becoming all the rage: easy-to-use libraries that can make your application look sexy. Here's a short list to help you out:
Moo.fx is the newest one on the block and boasts a small 3KB footprint.
Ignore for a minute that you need the 27KB footprint of Prototype to go along with that. All snarkiness aside, Prototype actually has a number of classes built into it that aren't required for what moo.fx needs. Be sure to remove anything that you don't need because 30KB seems quite large just for some animations.
Anyways, moo.fx excels at keeping it simple. There's a small set of nice animations like changing height, width and opacity of an object. You can even combine effects which is handy.
Script.aculo.us, like moo.fx, is built upon the Prototype library but differs by building on top of the AJAX features also available in Prototype. It has also found its way into the Ruby on Rails framework. For this reason alone, expect to see this library to stick around.
Rico has been around for awhile and did NOT need the Prototype library. But after discovering that they were duplicating a lot of the same work, the Rico team has reworked their stuff to work off the Prototype base. If Prototype had licensing costs, they'd be making a mint by now! (no, not that mint.)
The largest limiting factor to all of these libraries is browser support. Since they all rely on the Prototype engine, you're limited to Internet Explorer 6, Mozilla 1.7+, Firefox 1.0+ or Safari 1.2+.
With Web 2.0 in full swing, don't be surprised to see these libraries extended and others libraries find their way. Now, where did I put that mouse trailer script...?