From the MooFx website:

“moo.fx is a superlightweight, ultratiny, megasmall javascript effects library, to be used with prototype.js or the mootools framework.

It’s very easy to use, blazing fast, cross-browser, standards compliant, provides controls to modify any CSS property of any HTML element, including colors, with builtin checks that won’t let a user break the effect with multiple, crazy clicks. Optimized to make you write the lesser code possible, the new moo.fx is so modular you can create any kind of effect with it.”

What is really nice about this project is that it is built to be used in conjunction with EITHER mootools.js or prototype.js. Also, it is built by one of the main developers of the mootools.js framework, so you know its good.
Moo.fx

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