This one is more bit interesting with a long story. Being very interested I created my first gravity-simulation program as soon as was able to draw graphics. At this time it was Pascal. With very slow graphics. I even had plans to create space shooting game on top of it, but all was doomed because of the poor performance.
After about a year it was recreated in Delphi, with the same core but better performance, as Windows Forms graphics was used.
Last year I decided to port it to XNA Windows Application. It handle sprite based graphics unlikely the past versions.
And finally, the Windows Phone version! With better graphics effects, particle system and Multi-touch gestures controls. Still, nothing known as gameplay in it 🙂 Made purely for fun.
You just place Sun, and keep on adding new planets and asteroids to see how the gravity would work on them. It doubles the Fun when you add seconds Sun ! Now you understand why solar system mostly have only one Sun(Star).
UPDATED: Surprisingly, this is my top downloaded game.