Freespace 2
Gonna go buy a copy on Ebay...because the copy floating around on HOTU (home of the underdogs) is missing half of the audio, all the videos, and the extra 20levels.
Comparison: HOTU version is 700mb. Original Version was like 4gig.
When I get it tho, I'll put in zips somewhere you guys can get it. It is freely distributable, as it says in the installer licence. Many additional mods are made and still being made for it. The community is just so small though. Would be great for a LAN.