"MooTools is a compact, modular, Object-Oriented JavaScript framework designed for the intermediate to advanced JavaScript developer. It allows you to write powerful, flexible, and cross-browser code with its elegant, well documented, and coherent API."
Downloads (book + cheatsheet)
Unfortunately the one bad thing about MooTools is the lack of any kind of reading material other than the documentation on the site.
However with the miracles of Screen Grabbing and some crafty PhotoShop batch processing I've managed to build a little PDF version of the brilliant MooTorial on ClientSide's site.
...all praise goes to Aaron Newton at ClientSide, you are a genius!
...praise here goes to mediaVROG for updating Snook's original work, you guys rock!
