MooTools JavaScript Framework

From mootools.net

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. MooTools code respects strict standards and doesn't throw any warnings. It's extensively documented and has meaningful variable names: a joy to browse and a snap to understand.