It's hard to come across sim games that can stand up to the likes of Freespace 2 (in fact, i dont think there are
ANY sim games that can match Freespace 2 !

)
But , perhaps there are some that come close. Games that jumped out at me that seem to have similarities to Freespace 2 are such:-
Mechwarrior 4 series (Vengeance, Black Knight, Mercanaries (there is a MASSIVE mod for Mercanaries, and the multiplayer community for MW4 :Merc is alive and well)
Freelancer / Starlancer (They're good for a space sim but dont come close to matching FS2. Freelancers attempt at 'replayability' gets rather dull after you finish the main campaign)
The 'X' series isnt even worth putting up here because it stinks so bad. It took the struggling genre that is space sims and cut off its legs then beat it to death.
This said, Freespace 2 holds a monopoly on the genre (what little is left of it). According to highly acclaimed game reveiw site 'gamespot', Freespace 2 is ranked 407 out of 37,809 games. Thats pretty damn impressive for a game that came out in '01 and isnt even on the shelves anymore. I think Hard Light and its community should give themselves a pat on the back for contributing to the popularity of the game, and letting their friends and their friends friends and everyone they know about Freespace 2, and the fact that such a wicked game is now free.
Kudos to you all. And to sum up, check out those games i listed, if you arent into them. They're worth looking at.