Introducing Closure Tools. Google have released the pure-JavaScript library, apparently used for Gmail, Google Docs and Google Maps. It comes with a powerful JavaScript optimiser tool with linting built in and an accompanying Firebug extension to ensure the obfuscated code it produces can still be debugged. There’s also a template system which precompiles down to JavaScript and can also be called from Java.
Actually it was released long ago under the name of "doctype": http://code.google.com/p/doctype/source/browse#svn /trunk/goog
Now they've added tools and more code.