Attack Vector is a free tabletop game of space combat.
Rules: Science Fiction
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.
Silent Dark Science Fiction Rules
Silent Dark (direct download link) is a set of free wargames rules for “tactical” science fiction games.