MooTools – Compact, Modular, Object-Oriented JavaScript Framework
http://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 well commented and has meaningful variable names: a joy to browse and a snap to understand. This has been added Script Resources Subject Tracer™ Information Blog. This has been added to World Wide Web Reference Subject Tracer™ Information Blog.