Celestia is an open source program for exploring space. It isn't exactly a game, since there's no particular objective; you just zoom around and look at stuff and maybe learn something about astronomy in the process. You can travel to other galaxies, but it's based on real astronomical data so our own solar system is far more detailed than anything else out there. I'd been playing with it for a while, but I just realized how much better it looks with some of the alternate textures available from the Celestia Motherlode.
These screengrabs I took all feature the alternate textures.
Closeup of Ganymede. The small circle near Jupiter is Europa (I think).
Io's shadow is a black dot on Jupiter's surface.
The Hubble space telescope above the Sinai peninsula.
The International Space Station above the mouth of the Amazon river.
The ISS above Italy at night.