Building a Gravity Simulator. Full details can be found here: http://www.engr.uvic.ca/~liamk
Technically it's a study of Parallel Programming Techniques with a Focus of the Cell Processor, but the sample problem I'm cranking away at is a gravity simulator.
A sweet youtube video of it in action can be found here! Unfortunately recording it slowed it down by about half, so I suggest jumping to about 3 minutes in, or maybe just having it run in the corner of your screen while you do something else.