MooTools

MooTools is a compact, modular, Object-Oriented javascript framework based on the popular moo.fx javascript effects library. It is compatible and fully tested with Safari, Internet Explorer 6 and 7, Firefox and Opera, and the site offers comprehensive documentation of what it is capable of and how to do it.

Licenced under the liberal Open Source MIT Licence, you are free to use it and modify it in any way you like, free of charge.

Definitely worth a look!

» MooTools.net