MooTools' Fx libraries are both extensive and elegant and put to good use can really make a web site shine. To a great extent, everything you need is already there; anything you want to animate you can do with the Fx libraries in MooTools.
We've added a few libraries of our own, but really they aren't effects. They are widgets that extend Fx but they aren't themselves effects. Still, that's where we've decided to collect them.