There are a few interesting projects going on:
- Raspberry Pi + Lego based “supercomputer”: http://www.southampton.ac.uk/~sjc/raspberrypi/. It is estimated you need 1.4 million Raspis (and a lot of Lego) to build a machine that appears in the TOP500 of supercomputers (http://hackaday.com/2012/09/12/64-rasberry-pis-turned-into-a-supercomputer/).
- Adepteva (http://www.adapteva.com/) is working on a $99 board including some 16 parallel RISC coprocessor cores. “A cluster of 100 16-core Parallellas would cost $10,000 and provide 10 teraflops” (http://arstechnica.com/information-technology/2012/09/99-raspberry-pi-sized-supercomputer-touted-in-kickstarter-project/).