Scientists at the US National Center for Supercomputing Application (NCSA) are currently testing linked Playstation2 consoles in an effort to reduce supercomputing costs. The costs for this system are around $50K.
They currently have 70 of the consoles linked together and are using a Sony version of Linux as the OS. NCSA estimate that the consoles are capable of half a trillion operations per second. This compares with other supercomputers.
One of the limitations is the PS2s small amount of on board memory. PS2s have 32Mb. This limits the data transfer rate to the graphics chip which is the processor performing the crunching. http://news.bbc.co.uk/1/hi/technology/2940422.stm