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.
Miniatures Rules
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.
Swamp Fox American Revolution Campaign Game
The Perfect Captain’s Swamp Fox is a set of rules for American Revolution campaigns.
Hoplomachia Peloponnesian War Rules
Hoplomachia is a set of free wargames rules for the Peloponnesian War.