The Jackson Gamers have this set of free wargamesrules for the English Civil War. They use a card activation system, (not surprising, considering the acknowledged Brom influence), and d6s to resolve actions. These rules are 20 years old and in their 6th edition, so they are well-tested.
Matt Fritz has these simple set of free wargamesrules for recreating the Siege of Tenochtitlan. He uses them with his students — a great idea for both wargaming and learning! It inspires me to try similar things in my classroom.
Clif’s 1490 – 1690 Rules are the author’s free wargames rules for the English Civil War, 30 Years War and other conflicts of the period.
Kensi Samurai Heroic Skirmish is a set of free wargames rules based on Games Workshop’s Lord of the Rings Battle game.
The older I get, the more I like simple rules sets. Here’s one for Samurai Skirmishes.
From the introduction:
Sensō is a skirmish game, 1:1 figure scale, for 25mm miniatures. One scale inch represents six feet and one turn roughly approximates one-two minutes. Bases should be square for foot, approximately one inch. Mounted can be 1” x 2”. The game is WYSIWYG (What you see is what you get); figures are armed and armoured as they appear.