The Death Zap blog offers a set of free wargames rules for small unit combat. There’s also a fun battle report that goes with them.
modern
Cold War Naval Battles
Cold War Naval Battles is a free print-and-play version of the classic Modern Naval Battles card game. We played the heck out of MNB back in the day.
Fireteam II Modern Skirmish Rules
John Armatys offers a set of free modern skirmish wargames rules called Fireteam II They’re designed for use with 20mm figures.
Platoon Squad Level Vietnam Era Rules
Platoon is a set of free wargames rules based on the classic Avalon Hill game Platoon, which is named after the movie of the same name.
Point Blank Skirmish Rules
Point Blank is a set of free wargames rules where the units are fire teams and squads. Soldiers are represented individually.