Elite
 

There have been a couple of mentions of the BBC classic "Elite" on another thread - thought it might be worth pointing out that you can get hold of an open source version - Oolite - works on Mac OSX Linux and Windows.

As well as the basic missions included in the old version (Trumbles, constrictor, thargoid plans) there's at least one other mission and a bunch of extra weapons and features. You can also download OXP packs which contain user-created expansions to the game.