The Cluster HAT interfaces a (Controller) Raspberry Pi A+/B+/2/3 with 4 Raspberry Pi Zeros (NOT included). Configured to use USB Gadget mode, it is an ideal tool for teaching, testing or simulating small scale clusters.
Virginia Tech Apple G5 super computer cluster - YouTube
Build your own supercomputer out of Raspberry Pi boards Summary: Who says you need a few million bucks to build a supercomputer? Joshua Kiepert put together a Linux-powered Beowulf cluster with Raspberry Pi computers for less than $2,000. Steven J. Vaughan-Nichols By Steven J. Vaughan-Nichols for Linux and Open Source | May 23, 2013