Originally Posted by
Forward Observer
If you want something with a bit more meat, the most involved and dynamic sandbox space shooters are the "X" series of game. The latest is X3:Reunion. It's well over two years old now, but has been re-issued with quite a few updates and additional content over the original. (It originally came with Starforce copy protection, which was removed in the re-issue.) It's also available on Steam for direct download.
The graphics are far superior to Freelancer, but don't require anything outlandish to run, and another big difference is that you can eventually acquire multiple ships, hire pilots, and form squadrons of space craft. I believe you can even conquer and own whole planets.
Be warned though, it has a fairly complex learning curve and probably will require a joystick to fly combat missions properly. Still, everybody that takes the time to get into it simply raves about it.