Here is a simple, two page (front and back) set of rules for the 100 Years War by Eric Wood.
The Jackson Gamers present their Medieval version of the venerable Rules by Ral.
Lord Merlin’s Rules of Engagement is a set of free wargames rules for medieval style wargames. What makes these unique is that they are designed for large scale (70mm) figures. Despite the “Lord Merlin” monicker, there doesn’t seem to be a fantasy component here.
Mindstalkers: Mission Middle Ages is a set of rules for skirmish games in a medieval setting.
From the Perfect Captain comes Alexius, a campain game set in the Byzantine Empire in the time of the Crusades.