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.
Rules: Science Fiction
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.
Titan Rampant – SciFi Dragon Rampant Variant
Titan Rampant is a free variant for the commercial Dragon Rampant rules. Ownership or access to Dragon Rampant is required for play.
Free Project Z Rules From Warlord
Warlord Games is offering a free pdf of their Project Z zombie miniatures rules.