Planet 28 is a set of free wargames rules for “quick narrative games between warbands of warriors, mutants, and outcasts.”
Rules: Science Fiction
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.
Mecha!/Spirit Warrior Rules
For a limited time, Scale Creep Miniatures is making its classic Mecha!/Spirit Warriors rules available. These feature big stompy robots with an Aztec flair.