The Department of Defense used 1760 Playstation 3s to build a supercomputer because it was the cheapest option!
The United States Air Force Research Laboratory has completed what it calls the Condor Cluster, a supercomputer made entirely of PlayStation 3 consoles. The Condor Cluster, housed in Rome, New York, reportedly has capabilities heads and tails above every other interactive computer in the Department of Defense.
Made of 1,760 PlayStation 3 processors and 168 general purpose processors, the Condor Cluster provides extreme power for it's relatively low cost. It has the ability to calculate 500 trillion floating point operations per second (TFLOPS), but was made for $2 million thanks to the cost of the PS3.
Mark Barnell, director of the Air Force Research Laboratory's High Power Computing division, says this is "a cost savings of between 10 and 20 times for the equivalent capability." It also uses 1/10 the power of a comparable supercomputer, apparently making it "green."