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Closure Library is a broad, well-tested, modular, and cross-browser JavaScript library. Web developers can pull just what they need from a wide set of reusable UI widgets and controls, as well as lower-level utilities for the DOM, server communication, animation, data structures, unit testing, rich-text editing, and much, much more.
The source code is licensed under the Apache license and available at [code.google.com...]
Api docs: [closure-library.googlecode.com...]
It comes with a JavaScript optimizer, a set of templates, and a code inspector. I haven't had a chance yet to test the code, but from the description it looks pretty impressive.
