There's also Free Allegiance, a former Microsoft game but which miraculously they've abandoned and actually given to the fans to expand on and play, it's a bit complicated to set up since it's mostly online only (they've actually managed to pull off some games involving around 200+ people IIRC... It has real physics (meaning you don't stop after you've accelerated, you have to use counter-thrust, meaning also you can accelerate in one direction, spin around and fire backwards whilst still going in the original direction). Oh, did I mention it was free to download and play?
http://www.freeallegiance.org/
Also coming up as free to play space shooter MMO is Black Prophecy... is going to be awesome, I wish I had gotten to the pre-beta testing... oh well, guess I'll just sign on when it goes live...
http://blackprophecy.com/