To Cry A Joust is a set of free wargames rules for jousting combats.
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.