Mechbrawl is a free expandable card game. In it, you face off against your opponent in a giant, customizable war robot.
science fiction
Starship Trooper Rules
Kurt Bowker has written a set of free wargames rules set in the Starship Troopers movie universe. I’m not sure where you would get the miniatures to go with these — perhaps Ground Zero Games has something that could be used.
Dungeon Squad: Adventures in the 41st Millennium
Dungeon Squad: Adventures in the 41st Millennium is a RPG set in the 40K universe.
HALO Combat Miniatures Evolved
HALO Combat Miniatures Evolved is a set of free wargames rules based on the HALO science fiction franchise.
Solar Scourge Science Fiction Rules
Solar Scourge is a set of free wargames rules designed for 6mm science fiction miniatures.