Raum Krieg is set in an alternate history where Germany and the Allies battle it out in the skies with UFO Technology.
The Starcraft Tactical Miniatures Combat rules are based on the world of the classic video game. I suppose you could use miniatures from the board game to play.
Tacship is a set of free wargames rules for space combat by John Tuffley. He writes:
Tacship is a simplified fast-play rules system for simulating combat between manned space fighters. The game may be played on a hex grid map or on a plain table area by substituting inches for hexes and measuring turns in 60 degree increments (the same as a one hexside facing change). Fighter models should be mounted on stands, preferably with a one inch hex base, if a grid is being used.
Unbreakable is a set of free wargames rules for
a game of futuristic ground combat for 2 players. Each player assembles a platoon of crawlers, walkers, and powered armor squads and equips them with nukes, stasis rays, lasers, and more. Then the platoons meet in combat where they unleash terrible powers of overwhelming destruction. Fortunately, no one gets hurt. Thanks to amazing leaps in materials technology, the battlefield of the future will no longer be a dangerous place (for the combatants).