If you are a fan of NASA and space flight, try this sim.
This isn't a game - you'll need to brush up on the physics of space flight to fly the sim. This is, however, the best piece of freeware I've ever seen. Using add-ons, you can fly anything from fictional craft, to any of the US programs from Mercury to the Space Shuttle.
Thanks for sharing.
Here's another, about deep space.
Not flying, but a look from a different perspective.