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.
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For The Masses demo rules
Majestic 12 Games has these demo rules for their For the Masses Fantasy miniature wargames system. Here’s their blurb:
For The Masses is a complete fantasy mass-combat wargame system. But it is also much more: The flexibility of the system allows players to use any fantasy setting and armies to play out famous fantastical scenarios and ancient epic battles. Several sample army lists are included in the book but the real strength of For the Masses is the unit creation system. This system encourages players to be creative, use their imagination and have fun, which is the way wargaming should be.
100 Years War Rules
Here is a simple, two page (front and back) set of rules for the 100 Years War by Eric Wood.
Rules By Ral variant
The Jackson Gamers present their Medieval version of the venerable Rules by Ral.