Realm is a set of free wargames rules for large scale fantasy battles.
Flat Out War is a "universal" set of free wargames rules. They're designed to let players use any miniatures in any period. The underlying principle is that a miniatures abilities are determined by dice with different number of faces.
There's a very interesting thread over in the Redditt board games forum. The question was "What are your favorite games that take five minutes to set up and five minutes to explain."
It's a very interesting list, and there are quite a few games there I've never considered. My group often finishes one game early and is looking for another to finish out the night. There's a good many here for thought.
To End All Wars is a set of free wargames rules for squad-based WWI miniatures games. The authors write:
To End All Wars is a miniature game that attempts to accurately recreate the First World War. Players control teams which have different roles in combat. They use a combination of teams to take positions on the battlefield and force their enemy from the trenches.
Laser Grenadiers is a set of free wargames rules for science fiction games. The author writes
LaserGrenadiers extends modern combined arms combat into the future, allowing players to field powered armor infantry, tanks, crewed weapons, tanks, wardrones, helicopters, battledrones or “mechs,” artillery and ground attack aircraft. Vehicle propulsion systems may be wheeled, tracked, grav, hover, rocket or walker. The rules include all types of science fiction weaponry (lasers, cone rifles, phasers, ion cannon, etc.), but do not include psionic warfare. Fire combat dominates the game, with melee combat occurring infrequently. The rules were intended for use with any science fiction figures the players may have, and have been used for battles ranging from a few figures on each side up to the final battles of major campaigns with up to a hundred figures and twenty armored vehicles per side.