Freespace 2 is, in a word, awesome. Plus, you can download it free(legally) and there's a mod community improving it!
Even now. I’ll admit, I didn’t play the game when it first came out, but downloaded it now that the source code has been released to the community (you can download the entire game for free). Not only that, there is a mod community backing it that is really overhauling all the graphics and campaigns, which is refreshing. The game is incredibly enjoyable, from the first encounter with the shivans to the first time you play a bonus mission that involves you penetrating alien space. You won’t forget the “DIVE! DIVE! Hit your afterburners, pilot!”
There are a wide range of difficulties, from easy, in which you brush off flak without even damaging your shields, to hard, where anti-fighter beams are a real danger. The graphics are enjoyable, though I recommend changing the HUD color in the nebulae missions (its in the options).
The gameplay is great, it’s free, and there’s a mod community making it even better. Play it.