Dust Models offers a free digital version of their Dust 1947 rules.
Rules: Science Fiction
Clobberin’ Time Superhero Skirmish Rules
Clobberin’ Time is a set of rules for fast superhero skirmishes.
Attack Vector Space Combat Game
Attack Vector is a freeĀ tabletop gameĀ of space combat.
The Stars Our Decimation: Voidstriker
The Stars Our Decimation: Voidstriker is a set of fleet oriented starship combat rules. They can be used as a board game, and with miniatures and hexless play.
Mobile Frame Zero Lego Rules
Mobile Frame Zero is a set of free wargames rules (although you can pay, too, if you like it and can get a printed version) for playing games using Lego and other construction toys. It is very well done.