Closure, which initially started as a 20% project(Google employees use their 20% free time to innovate on their own) has come a long way and grown to be an integral part of major Google apps. The Google Code blog announcing the release, said:”
We want to hear what you think, but more importantly, we want to see what you make.
The source is available from Google Code and is distributed under the Apache License.
For download of the Closure Library, visit this page.