Star Breach is a set of science fiction rules that are miniatures agnostic. The free 1st edition core rules are available at the link. There’s also a deluxe version you can buy.
science fiction
Planet 28 Sci Fi Skirmish Rules
Planet 28 is a set of free wargames rules for “quick narrative games between warbands of warriors, mutants, and outcasts.”
Warpath Rules
Mantic offers its Warpath science fiction rules free for the downloading.
Deadzone Rules
Mantic offers a free pdf version of its Deadzone science fiction rules. I played these at a convention in recent months and enjoyed them very much.
Laserblade Science Fiction Skirmish Rules
Laserblade is a set of free wargames rules for science fiction skirmishes.