One of my many hats is security, which means sometimes I need to check out a system's passwords to see if perhaps somebody set something stupid. Since it's far more effective to tell somebody "your stupid password abc123" (which I've now seen in compromises twice!) than it is to ask them what it was (sometimes they won't admit to it), I've occasionally had need to bruteforce them.
Somebody in #pauldotcom pointed out that there are MPI patches for john the ripper, which makes me happy; now I have something I can use my test cluster for. There's also a list of tools here and Muts has been working on adding a cluster setup for john to Backtrack. (The first and the last may be related; I've not had the chance yet to check them out thoroughly.)