WarZone, in its original incarnation some twenty years ago, was one of my favorite games. I still have substantial painted armies from that era. Then the game went through a series of owners, rules changes, and model changes. Eventually, it died, only to be resurrected by Prodos. That company has the latest version of the WarZone rules free for the downloading.
Rules: Science Fiction
Frag-O-Rama Skirmish Miniatures Rules
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.
A Fistful of Sardaukar Dune Miniatures Game
A Fistful of Sardaukar is a set of free wargames rules for battles in the Dune universe of Frank Herbert.
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.