PZ8 Sci Fi is a set of free wargames rules for using 6mm (microarmor) figures in a science fiction setting. They’re available on the Yahoo 6mm Wargames Group.
PZ8 1950-69 is a set of free wargames rules for using 6mm miniatures to refight battles of the mid to late 20th century. They’re at the 6mm Miniatures Yahoo Group.
Ironhands has a page with tips for creating sufficiently alien Eldar terrain for your Warhammer 40K games.
Counterterrorist Force is a set of free wargames rules for gaming modern counter terrorist operations. The author writes:
CTF (Counter-Terrorist Force) is a pure “player vs. player” (1 on 1) game where the main focus is on tactics and realistic combat. The game is played from both sides of the conflict. One person plays the terrorists and the other takes the role of counterterrorist. Not only do you execute missions, you also run your own counter terrorist corporation and try to work your way up from scratch. You will also fight occasionally as a terrorist so that your friends can play as counter terrorist troopers too. Game play can either be single missions to improve your corporation, or otherwise in campaign mode where two campaigns are interwoven – you and your opponent take turns as terrorists – and you both have your own counterterrorist unit to run every second game in your individual campaign.
Fleet of Battle is a set of free wargames rules for playing out battles between fleets of ancient galleys. Think Ben Hur. There also are several pages of free print-and-play galleys for use in lieu of lead miniatures. The rules require a knowledge of the commercial Field of Battle wargames rules.