This guy is building a cluster of low-cost PIC microcontrollers to experiment and learn with parallel computing design.

Wanting to experiment and learn more about the design variances involved when using a large number of processors vs. a small number, Peter Jansen is building a board which houses 10 dsPIC30F3013 microcontrollers all arbitrated by an Imsys SNAP module. The SNAP module is basically a self-contained Linux computer with networking, flash memory interfaces and other high-level goodies built-in.

When you're using that many processors, designing the interconnect buses and communication protocols can get to be a challenge - at least if you want the chips to be able to do anything in an organized manner. The concepts in this project will be similar to the large-number of simple processors design used in the Playstation3.

Check-out Peter's PIC cluster page for more information. It's still a bit light on details but the project is still under development.