This is the perfect space combat game. When I first discovered it, copies were going for $100+ on ebay. Buy this game if you're a space combat fan. For $6, buy it even if you aren't
As others have mentioned, the game engine's source code was released as the Free Space Open project. There are some stellar mods (BSG and Babylon 5) and some top-notch new missions. The people on the project have even redone the graphics. FS2 still looks pretty good in 2008, FSO looks fantastic. (Though X3's graphics are nicer. Pitty the game-play isn't as good as Freespace.)