Dojo saves you time, delivers powerful performance, and scales with your development process. It’s the toolkit experienced developers turn to for building superior desktop and mobile web experiences.
MochiKit is a highly documented and well tested suite of JavaScript libraries that will help you get things done, fast. It takes good ideas found in Python, Objective-C, etc. and adapts them to the crazy world of JavaScript.
The YUI Library provides tools for professional web development, including a rich JavaScript and CSS library -- all freely available under a liberal BSD license.