I guess the parallelism is why botnets are so good at it. I wonder if anyone's tried getting JavaScript web workers (or possibly, somehow WebGL) to poll a central server to do cracking. A site with just a few thousand visitors a day should be able to crack passwords faster than this machine.
I have attempted this, and I believe others on the wider Internet have as well. Essentially it ended up being slower than if the central servers CPU was used to attack the hashes instead. With only JavaScript available to do the hashing, the maximum per client was disappointingly slow, around 2000 hash/second each if I remember correctly.
It might be better now with faster JavaScript engines, but I wouldn't bet on it.
