Games Workshop offers the core rules for its Age of Sigmar Rules free for the downloading.
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.
Kampfgrupp OST is a simple Hex based Operational level WWII game circa 1944-1945 East front.
Pacwar is a set of rules for large scale dogfights in “The Universe.”