Pacwar is a set of rules for large scale dogfights in “The Universe.”
science fiction
Forge of War Science Fiction Rules
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.
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.