Net Epic is a set of free wargames rules based on the old Games Workshop Second Edition Space Marine.
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.
The Starmada demo miniature wargaming rules are available from Majestic 12 games:
Starmada is a simple (but not simplistic) board game of starship combat, in which players square off in a general attempt to blast each other’s fleets into just so much space junk. While the universe of the Imperial Starmada and its major adversaries is fully detailed in the rulebook, Starmada is much more than just another game, tied down to its own background. Instead, Starmada can be played in just about any science fiction setting you can imagine. There are dozens of optional rules, exotic special equipment items, and menacing weapons that can be used or discarded at your discretion, to create the type of game you want to play!
Rules Crusade is a complete rewrite of the Warhammer 40K, Mordheim and Necromunda wargames rules to use the old Milton Bradley Space Crusade system. Its well done, but I’m not sure why Sean Patten went to all the effort. I’m also not sure why GW’s hordes of lawyers haven’t descended upon him and turned him over to the Inquisitors.
Taktika is a set of free wargames rules for science fiction battles.