Forge of War is a set of free science fiction skirmish rules from Craig Cartmell. He writes that they were born of frustration with the Warhammer 40K rules from Games Workshop.
science fiction
Generic Outlandishly Big Spacefleets (GOBS) Rules
Generic Outlandishly Big Spacefleets (GOBS) is a set of free and easy-to-play rules for huge fleets of starships. The author writes:
G.O.B.S.! is fast-paced and easy to learn. Record-keeping is minimal; in most cases, ships blow up rather than retaining damage. (A two ship game of G.O.B.S.! would be over pretty quickly. This game is about fleets, Sparky! We want explosions all over the place!) The length of playing time is dependent on the size of your fleets, but an hour or two should do it for most cases. Wanna play longer? Build bigger fleets!
Mechwarrior Dark Age Rules
Mechwarrior Dark Age is a discontinued product of WizKids games. You can still find the figures on Ebay, though. Rules and Unit stats can be found at the links.
Making Laser Defense Batteries
The Iron Hand site has a tutorial on building a very nice building-sized laser defense battery for 25mm – 28mm science fiction games, such as Warhammer 40K. These would be great as the focal point of a scenario.
Galactic Heroes Star Wars Rules
Galactic Heroes is a set of rules for playing game set in the Star Wars universe. The author uses those chunky three inch “Star Wars Galactic Heroes” figures, but any would work well. The author writes:
Galactic Heroes is a friendly game for 2-6 players, ages 6 and up. Each turn, players move their action figures around the table, exploring by flipping over Tokens that could be an enemy, a trap, or even something valuable. When a player successfully deals with an encounter, they collect the token. When all the tokens are collected, the game ends and the player with the most tokens wins!