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.
Rules: Science Fiction
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.
Corvet: Skirmish Starship Combat
Corvet is a free skirmish game of starship combat for two or more players. What makes it different is that it tries to implement a “captain’s-eye view of your ships”
Things – Rules For Horror Skirmishes
Things is a set of free wargames rules for science fiction / action / horror skirmishes, inspired by movies such as “The Thing.”