A number of weeks ago I discovered this game and promptly posted it on the website under free games. Its absolutely amazing and Im going to be playing it for a LONG time. Its almost 10 years old yet has been modernised over and over. It will run on old machines but still look brilliant on modern ones. I dont know how to explain how great it is to play nor how brilliant the community is. One of the total conversion is a Battlestar Galactica one which won best indie game of 2007 at moddb. There are also Wing Commander, Babyloon 5, Starfox, Halo and Homeworld full conversions. They have also ported the entire FreeSpace 1 game to the new engine.
The user made campaigns are so well done they have voice acting, something most mods never have. User upgrades to the engine include normal mapping, experimental voice recognition... its all staggering. And to top it off as a free open source product is amazing.
There is so much content I bought one of these just to play it on:

I cannot recommend this game enough.
http://www.youtube.com/watch?v=-6VhyMzu12c