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.
One Man One Knight (OMOK) is a set of free wargames rules for medieval skirmishes.
Battlestorm is a set of free wargames rules for medieval fantasy.
Andy Watkins offers a set of skirmish rules for the Wars of the Roses. Watkins writes:
25mm Skirmish wargaming with Wars of the Roses figures was one of the first Skirmish games I ever played about 20 years ago. To be honest I haven’t played much for quite a while. The rules I used were a bit cumbersome, I’ve upgraded now to a home grown set that owe more to my Pirate rules than anything else. Though there is still a touch of influence from Retinue, the best of the commercial Wars Of The Roses rules.