http://www.microsoft.com/games/freelancer
http://www.lancersreactor.com
I recommend you to try Freelancer, you can buy it for 5-10$.
Freelancer is a Space Simulation with RPG elements.
It's a kind of MMORPG on player-hosted servers + SP Mode.
The action is endless as you make your way through 48 known star systems and additional systems, spaceships, weapons made by Mods.
Missions, your successes, your failures, they all change your reputation constantly.