All Quiet On The Martian Front is a great game set in an alternate history where HG Wells’ Martians return in 1908. It’s got a WWI vibe, with steam tanks and big cannons and motorcycles. You can take a look at the free rules here.
Zombie Escape is a Warhammer 40K modification for Imperial Guard versus Zombies
PZ8 Sci Fi is a set of free wargames rules for using 6mm (microarmor) figures in a science fiction setting. They’re available on the Yahoo 6mm Wargames Group.
Pacwar is a set of rules for large scale dogfights in “The Universe.”
Forge of War is a set of free science fiction skirmish rules from Craig Cartmell. He writes that they were born of frustration with the Warhammer 40K rules from Games Workshop.