moo.fx is a lightweight, megasmall, easy to use, fast, cross-browser, javascript effects library, written with prototype.js.
It’s a real blast and I can’t wait to use it in a project or two. In the meantime I recommend a look at these two tutorials for beginners, they’re really well written and straghtforward.

moo.fx tutorial quickie
The Idiot’s Guide to moo.fx

Have fun. :)