Universe Sandbox. Bought it on steam for around 10 euros. (http://universesandbox.com/)

It's a universe simulator. You can do whatever you want with it. I can't tell how accurate the physics is, but it seems solid to me.

I'm not sure if it really qualifies as a game, since there isn't any goals or achievments, no resources to mine, no prioduction, no research trees. No enemies. It's about exploring and manipulating stars and planets from a distance, and then lean back to see what happens. Which to some of us (me) is satisfying in itself. I had great fun watching how the solar system got shredded by increasing the mass of Jupiter to 0.5 solar masses.

Now, back to reading the phone directory.