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.

Check out demos here 

mootools Version 1.3

Documentation overhaul (fixes and enhancements)

• Enhancements
  • Fx.Morph
  • Sortables
  • Selectors
  • More Element Extensions
• New
  • SVN Structures
  • download builder
  • MooTools Specs
• Bug fixes, optimizations, and enhancements
  • Better Safari 3 integration (Leopard Safari)