The Simple Toy Soldier Game is a set of free wargames rules from the Death Zap blog. It is designed for quick skirmishes with 4 – 8 figures per side and modern weaponry.
The Wayback machine has stored a copy of the WRG 1925 to 1950 armor and infantry rules from 1973.
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.
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.
John Armatys offers a set of free modern skirmish wargames rules called Fireteam II They’re designed for use with 20mm figures.