Harry Pratt’s Battlestar Galactica Starship Combat Rules are designed to recreate starship combat rules from the msot recent Galaxtica television shows.
I have to say that the fleet combat sequences in the series are absolutely spectacular, with those missles streaking through space and the ships burning up under multiple hits. Perhaps not the most scientifically valid, but spectacular nonetheless.
I absolutely love the Full Thrust starship rules. If you can’t get enought of the rules, there’s now a Full Thrust Wiki.
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.
Iron Cow is a set of simple rules pitting companies and battalions of science fiction and armor against each other.