The Closure Library is a broad, well-tested, modular, and cross-browser JavaScript library. You can pull just what you need from a large set of reusable UI widgets and controls, and from lower-level utilities for DOM manipulation, server communication, animation, data structures, unit testing, rich-text editing, and more.

The Closure Library is being used by the following: Google products: Gmail, Maps, Docs, Sites, Books, Reader, Blogger, Calendar, Photos

The Closure Library is server-agnostic, and is intended for use with the Closure Compiler.

jQuery is a fast and concise JavaScript Library that simplifies HTML document traversing, event handling, animating, and Ajax interactions for rapid web development. jQuery is designed to change the way that you write JavaScript.

Cappuccino is an open source framework that makes it easy to build desktop-caliber applications that run in a web browser. It is written with Objective-J which is JavaScript version of Objective-C

A toolset rather than a Javascript library, it serves similar purpose of creating interactive applications running in the browser.