Anyone who hasn't seen the wonderful things the Freespace 2 Open Source team has done over the years should really check it out. The game itself is $6 on GoG, and since Volition released the source code it's seen a complete overhaul for everything from AI to graphics. The classic campaigns are still there (and are well worth playing through), and there's been some really great fan campaigns released over the years too.
I'd highly recommend the Blue Planet campaign. Part 2 (War in Heaven) is especially noteworthy for really giving you the feeling of being on the losing side of a war, something you don't see very often in games. Part 1 has full voice acting, with voice acting for part 2 still in progress.