Google has open-sourced the javascript library and optimizer they use in gmail and other web applications. They call it "Closure" and you can read about it here.
While the optimizer looks very cool, I think the library is most impressive. Its a really large code base, and at least some of it has been thoroughly field-tested in gmail and similar applications. You can get the code like this:
svn checkout http://closure-library.googlecode.com/svn/trunk/ closure-library-read-only
The closure library is intended to be used with the closure optimizer which removes unused code, so the APIs are broad with lots of methods--there is no sense that the closure developers were skimping on API in order to pack everything into a small download bundle. Also, and perhaps for the same reason, the code is not full of micro-optimizations. Compared to the jQuery and YUI code (for example) the Closure code is straightforward and easy to understand.
@__DavidFlanagan
