Wing Commander Saga is a free game based on the Freespace 2 Source Code Project. It is based upon the popular Wing Commander series by Chris Roberts and Origin, and features a new story line during the final portion of the Terran Confederations deseprate fight for survivial against the Kilrathi Empire. You assume the role of 2nd Lieutenant David Markham, a recent OCS graduate in an accelerated flight training program. Victory and survival for humanity can only be bought at a terrible cost. Gather your wingmen and your courage and prepare to play your part in humanity's desperate gambit as you face the Kilrathi foe. It is a time where even average men must reach inside themselves and become heroes. Answer the call. Face the enemy. Make your mark as the Confederation faces its darkest dawn.
Wing Commander Saga is probably one of the most polished indie releases I have ever seen. I have only played through about half of the campaign(I think) but so far it has had a large variety in the mission structure with very solid combat mechanics. The characters and voice work can be spotty at times but overall it is superb especially considering the sheer volume included in the game.
Any Wing Commander or Space combat sim fan would be silly to miss out on such an amazing free release.