Fearless is a set of free wargames rules that are:
… intended as a fast-paced science-fiction tabletop game including role-playing elements, with a universe full of unknown possibilities as the setting. It allows you to guide mercenaries from various backgrounds into breath-taking fights in the Arenas of Fearless. Additionally prepared scenarios and the possibility to customize your characters offer even more fun to play the game.
Gene Autrey’s serial, The Phantom Empire is available for viewing online. It’s a great mix of cowboys and science fiction:
From IMDB:When the ancient continent of Mu sank beneath the ocean, some of its inhabitant survived in caverns beneath the sea. Cowboy singer Gene Autry stumbles upon the civilization, now buried beneath his own Radio Ranch. The Muranians have developed technology and weaponry such as television and ray guns. Their rich supply of radium draws unscrupulous speculators from the surface. The peaceful civilization of the Muranians is corrupted by the greed from above, and it becomes Autry’s task to prevent all-out war, ideally without disrupting his regular radio show.
Stargrunt 40,000 is a system for converting Warhammer 40K army lists into a format usable with Stargrunt.
Blasted Planets is a set of free wargames rules for science fiction skirmishes.
Blasted Planets Skirmish, is basically a set of house rules a few friends and I use to play games with my Sci-Fi figures. I am not trying to model a “Hard” Sci-Fi environment here. I’m borrowing as much from Horror and Fantasy as I am from Sci-Fi. I mean how serious can you be. When you have, Dog headed, Gator men chasing an Elephant guy around an Omni Field generating White rabbit? There is enough “real” world framework here that basic tactics work (most of the time). But enough of a fantastical element that you really can’t count on the idea that the Fin headed alien with the pin gun is going to nail the cave man with the rock every time.
Afterglow Skirmish Warfare In The Far Future is a set of free wargames rules for 28mm figures and is designed to be used with a variety of manufacturers.