Well, it doesn’t run in the browser, but it keeps itself to a single directory and runs from the command line, which is the next best thing. The GIMPS project has been around since 1996 and harnesses the “power of 1000’s of small computers like yours” to discover new Mersenne primes, which are primes of the form 2p-1. 46 Mersenne primes have been found, the largest having 12,978,189 digits.
The project was featured on NPR recently. There are prizes to win for discovering the next prime (give it a few hundred Ghz-days and I might pay for the Mac Pro with this), but best of all, it doesn’t do creepy stuff like create groups and users on your system. You just start it up the executable from a command line when you’re in the mood to hunt some primes and heat up the office. Finally something to keep the Mac Pro’s CPUs busy: