A Fistful of Sardaukar is a set of free wargames rules for battles in the Dune universe of Frank Herbert.
Rules: Science Fiction
No Limits Science Fiction Rules
No Limits is a set of free wargames rules for tabletop battles in science fiction universes. Any miniatures from any game can be used.
One Page 40K Rules
One Page 40K Rules are a set of simple rules for the 40K Universe. The authors write:
One Page 40k is a single-page ruleset designed to be fast to learn and simple to play. It serves as a great introduction to the Warhammer 40.000 universe for people who are new to the game, and as a way to play quick games for veterans of the franchise.
Fast and Dirty Sci Fi Rules
Looking for an alternative to Warhammer 40K? Who isn’t? This may be it. Even better: these wargames rules are free. Ivan Sorensen’s Fast and Dirty rules also would work for modern combat.
Kryomek Science Fiction Rules
Scotia Grendel now offer their classic Kryomek science fiction skirmish rules as a free download.