Script.aculo.us provides you with easy-to-use, cross-browser user interface JavaScript libraries to make your web sites and web applications fly. You can implement different modern effects easily, for example Fade in/out, Blind in/out, Drag and drop, Slide in/out, highlight, AutoComplete and Sortable List, etc. It is one of the most popular animation framworkm which is really powerful and similar to Mootools, however the size of the package is larger than Mootools. Here is the sample code.

new Effect.Opacity(’my_element’, { duration: 2.0, transition: Effect.Transitions.linear, from: 1.0, to: 0.5 });

Requirements: Required Prototype
Demo: http://wiki.script.aculo.us/scriptaculous/show/Demos
License: MIT License