Normally an Arduino project consists of only one Arduino and maybe some supporting electronics. I find Arduinos performing very well for what you pay for them so I thought: what if I create a beowulf-alike cluster of these? ("imagine a beowulf cluster of ..." was a meme on Slashdot.org for a while).
The idea behind this project is to create a cluster of arduinos (pro minis running at 3,3V) that calculate the mandelbrot fractal, directed by a raspberry pi. The arduinos here function like a cluster of math co-processors for the raspberry pi. The raspberry pi displays the result.
The Arduinos are connected to the Raspberry Pi via the i2c bus. They get their power via the 5V pin of the RPI (can deliver enough current for 4 arduinos).