JQuery: http://jquery.com/
Prototype: http://www.prototypejs.org/
Prototype is a JavaScript Framework that aims to ease development of dynamic web applications.Featuring a unique, easy-to-use toolkit for class-driven development and the nicest Ajax library around, Prototype is quickly becoming the codebase of choice for web application developers everywhere.
MochiKit: http://mochikit.com/
MochiKit is a highly documented and well tested suite of JavaScript libraries
