Clobberin’ Time is a set of rules for fast superhero skirmishes.
Rules: Science Fiction
Attack Vector Space Combat Game
Attack Vector is a free tabletop game 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.
Earth Expects Space Combat Rules
Earth Expects is a set of free wargames rules for space combat. The authors write:
Earth Expects is a highly detailed set of tactical space combat rules which emphasise skill over firepower. It is well suited to playing battles from duels to squadron actions; because of the high level of detail large fleet battles can be slow to play and therefore optional rules are provided to speed play in these games.