Check out this mod of the superb Freespace 2 engine (thank you thank you thank you for releasing it as open source Volition!). It’s a three level demo and man is it great – they’ve done a fantastic job with the voicework and they’ve made enough tweaks to the engine that it feels greatly different from the default Freespace 2 combat. Right now only the PC version is out but Linux and OSX are coming soon, and here’s hoping the full game is as well. This is free, open source, and a self-standing installer.