Here are two projects that use raspberry pi’s as the basic units of a supercomputer:
A 64 node cluster, build using pi’s and lego, built at the University of Southampton.
A 32 node cluster, build using pi’s and more standard computer pieces, built at the Boise State University+
<figcaption class="wp-caption-text">Beowolf Raspberry Pi cluster. Image from http://coen.boisestate.edu/ece/raspberry-pi/</figcaption></figure>
BeagleBone Black cluster by Dan Ricart