Marcus Reynolds offers Warriors of the Wasteland, a set of skirmish level science fiction rules that he devised when he became tired of the shenanigans of a certain miniatures gaming company who shall remain nameless but whose initials are GW.
Fallout is a legendary computer role playing game. Set in a post nuclear world, it has a 1950s science fiction aura. Here’s a miniatures game set in that world.
Jon Clarke has writtenUrban Myth , a set of free wargames rules set in a kind of a near future cyberpunk setting. What makes these rules stand out is in the basic die mechanism. It uses a d100 system (rolling two ten-sided dice), but you read the dice differently depending upon the difficulty level of the task:
All rolls are on d100 needing to get equal to or less than the Attribute being tested against.
How the dice are read depends on the difficulty of the test. If the action is easier it may be reduced one step or more, if harder increased one step or more.
There are three levels of difficulty for any task:
Easy: roll d100, read the dice the worst way in relation to the task (generally speaking you read the dice to fail, but if they succeed either way place the lowest dice as tens)
Normal: roll d100, identify one dice as tens & one as units before rolling.
Hard: roll d100, read the dice however you like.
Scavenge Skirmish Survive is a set of free wargames rules for a tabletop game set in a post-apocalyptic volcanic winter. Link goes directly to a pdf
Michael Taylor has written Road Warriors, a set of free wargames rules that uses Matchbox cars for games set in a post-apocalyptic future, ala Mad Max.