http://orbit.medphys.ucl.ac.uk/orbit.html
This program is free, but it isn't a "space game". It's a fascinating combination of spaceflight simulation and a virtual tour of our solar system.
The designer, Dr. Martin Schweiger of the University of London, has given us a fascinating glimpse into what is "out there", and a means to get there, by piloting various spacecraft.
If you wanna literally fly to the moon, you can do it, but the learning curve is very steep.
Nevertheless, I highly recommend it.
Here's a sample picture: this is Europa and Jupiter
