I have X3 and Terran Conflict on my Steam, I need to sit down one of these days and really get into them.
As for space sims in general, I am rather fond of Kerbal Space Program. Just like it sounds, you run a space program, designing rocket ships for your alien astronauts....Kerbanauts...and seeing what the results are. Will you go to the "Mun", or other planets in the solar system like Duna? Or will your crew simply become a speck of carbon on the ground smashed into it at the mercy of the Newtonian physics?
It's still technically an alpha build, 0.21, but for an alpha it has the feel of a very completed game, can easily be compared to retail level, but constantly expanding.
http://www.kerbalspaceprogram.com
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