Thanks to Facebook, this game is becoming popular with my colleagues:
The goal is to see if you can create a stable planetary system that includes, not just a few piddly Earth-mass planets, but giant ice planets and stellar companions, as well. Stable, in this case, means the system lasts at least 500 years without any planets crashing. It helps if you know a bit about Newtonian mechanics, but even if you know nothing about physics, you can gain some intuition just by playing the game. (Read more about the game here.)
So, how many planets can you cram into two Earth orbits?