Frag-O-Rama is the "free miniatures game of multi-player, shoot-�em-up, videogame-style action for 2 to 10 players." It's also got a blog for discussing the rules development.
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A Fistful of Sardaukar is a set of free wargames rules for battles in the Dune universe of Frank Herbert.
No Limits is a set of free wargames rules for tabletop battles in science fiction universes. Any miniatures from any game can be used.
The first issue of Ares Magazine featured a game called "WorldKiller, The Game of Planetary Assault." There's also an interesting article on the "most significant science fiction and fantasy games" by (among others) Greg Costikyan. It's a nice little piece of history and a glimpse to what was state of the art nearly forty years ago.