This is so neat! A few years ago, the Oak Ridge National Lab created "Tiny Titan", a demonstration of how supercomptuers work using a cluster of Raspberry Pis. It's a scaled down model of the Titan supercomputer. To visualize what each note of the cluster is doing, blink(1) USB LEDs are plugged into each Raspberry Pi, indicating what its up to.
(photos courtesy of reddit user "0110010001100010" in this post)