Here are two projects that use card sized computers as the basic units for computing clusters:
A 64 node cluster, build using pi’s and lego, built at the University of Southampton.
Boinc is a platform for volunteer computing. Briefly, volunteer computer is a system where computer processor’s idle time (those periods where your computer is on, but not being used for anything) is turned into calculation time via a custom written software.
Parallela, an open source, open access card sized supercomputer, has the mission of bringing parallel computing to the masses by combining multiple RISC processors and very low power consumption. Produced by the Adapteva company.