The AD Wars is a set of free wargames rules that are adaptation of Warhammer Ancient Battles for science fiction games. It’s interesting. You will need a copy of Warhammer Ancients to play, though. But there are a lot of copies of that out there.
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Rules: Science Fiction
FUBAR One Page Modern / Science Fiction Rules
Fubar is a good looking one page set of modern and science fiction rules.
Free 3D Terrain For Dust Tactics
Top Solitario, one of my favorite sites for its quality free paper products, has a selection of 3-D terrain pieces for Dust and other games which can be downloaded, printed and quickly constructed — a bonus is that they also fold flat!
Aquazone Game of Deep Sea Exploration and Combat
Aqua Zone is a set of free wargames rules for playing miniatures games that feature underwater combat with submarines, DSV’s and marine animals.
Faction Wars Science Fiction Skirmish
Fire and Forget Games has released a downloadable PDF version of the rules for Faction Wars, the sci-fi skirmish game. The free 40-page PDF contains the basic rules of the game, sample squads for two factions, and a couple of scenarios to get you started.