Back when home computers were first invented a space combat/trading game called Elite came out on the BBC micro and spawned versions on to every other platform around at the time. Now you can relive your childhood (if your over 40) with oolite which is basically the same game but with modern tweeks and improvements and better still you can do it for free as it under GNU public licence -
Oolite
What's great about it is you can download ad ons to your own taste to add missions and ships you want to the game.