Sword and Dagger are a set of free wargames rules for medieval skirmishes. They’re very straightforward and at 11 pages, probably good for beer and pretzels games.
rules
Fistful of TOWs Introductory Rules
Ty Beard offers an introductory set of his Fistful of Tows rules for modern warfare.
Solitaire Song of Blades and Heroes
Dougi has a set of adaptations for solitaire play of the popular commercial set of rules Song of Blades and Heroes.
The Square Brigadier Rules
The Square Brigadier is a set of horse-and-musket rules based on Bob Cordery’s Chess board game inspired by Joe Morschauser.
Battlelore Napoleonics
Battlelore Napoleonics are a set of adaptations of the Battlelore Fantasy/Medieval card driven miniatures/board game. I like the Battlelore system a lot.
Beware: this site has an obnoxious pop-up video ad embedded in it.