The Space Simulator genre is defiantly one of the best and under appreciated out there.
One little gem that I play to death occasionally is Escape Velocity: Nova - a game that went under everyone's radar most likely...
And another which is still in Beta 0.5.0 'NAEV'. It seems to be coming along nicely, the devs have taken a lot of inspiration from Escape Velocity while improving on a lot. It's also free
Any of you guys have a favorite?