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.
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.
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.
Beyond The Gates of Antares Quick Start Rules
Warlord Games offers a set of free quick start rules for their science fiction small unit game, Beyond the Gates of Antares.
Cybercity Blues RPG
Cybercity Blues is a free RPG that has a lighter take on the cyberpunk genre. The authors write:
“Cyberblues is a bit like Cyberpunk, only more mellow.”
Cyberblues City is a near-future, action-adventure roleplaying game. Fast-playing and often funny, it offers a lighter take on the cyberpunk genre, both in terms of system and tone. Cyberblues City uses a much modified version of the Fudge roleplaying system designed to make it easy on GMs. And best of all, it is absolutely free.