Prof. Simon Cox has written an awesome tutorial about turning multiple RaspPis into an "Supercomputer Cluster". He himself build an cluster of 64 RaspPis at the University of Southampton. I could not resist but rebuild his project - with the 3 "old" (256MB) Pis I got. It turned out to be an quite awesome and interessting experience, introducing myself to MPI (Message Passing Interface)  for the first time. You should definitly take a look yourself.

http://www.southampton.ac.uk/~sjc/raspberrypi/