From the 'Open Source Development' files:
Google builds a lot of its own tools as part of its development efforts. Sometimes we get all 'get lucky' and the tools become open source and available too.
That's kinda cool, but what's even more impressive in my opinion are the usage mechanisms that Google is making available for the Closure Compiler. In addition to the command-line they've also got a Firefox extension that works with their Page Speed optimization tool.
That's one heck of a powerful tool to have.