When we are coding with javascript especially with some cool effects, e.g. Fade in/out, slide up/down, drag and drop effects, and etc… We need a javascript framework to allow us doing simple Ajax request and all these cool effects easily. MooTools lets you get the job done efficiently and effectively. It is a compact, modular, Object-Oriented javascript framework designed to make writing extensible and compatible code easier and faster. Compared to Prototype 1.5 + Scriptaculous (excluding unittest/controls) 60800k, MooTools is only 34165k.

Requirements: Safari, Internet Explorer 6.0+, Firefox, Opera and Camino
Demo: http://demos.mootools.net
License: MIT License