Legends of Old Mars is a Edgar Rice Burroughs’ Barsoom variant of Games Workshop’s Lord of the Rings / Legends of the Old West game. I think that system is perfect for playing games involving John Carter, Tars Tarkas and the other denizens of Burrough’s Barsoom.
Marcus Reynolds offers Warriors of the Wasteland, a set of skirmish level science fiction rules that he devised when he became tired of the shenanigans of a certain miniatures gaming company who shall remain nameless but whose initials are GW.
Here’s a neat set of science fiction logos from the past 40 years worth of television and movies in font form. That’s neat, becuase it allows you to scale them up or down as needed. Very useful for science fiction games.
Unbound: Endless War is a free print and play game for two p layers in which you build a map and attempt to take over a players’ bases. It’s a pay-what-you-like game.
Iron Cow is a set of simple rules pitting companies and battalions of science fiction and armor against each other.