Here’s a fun looking set of medieval skirmish rules called Bashing the Bishop. The title tells you just how serious they are.
Rules: Medieval
Tree of Battles 14th – 15th Century Wargames Rules
Tree of Battles is a set of free wargames rules for battles of the 14th to 15th Centuries.
Castles, Cogs & Chevauchées
Castles, Cogs & Chevauchées is a set of campaign style rules for skirmish level combat using the Knights and Knaves skirmish rules.
Knights and Knaves rules at the same link
Atteint! Jousting Rules
Atteint! is a set of free games rules for medieval jousting.
Border Reivers Rules
Border Reivers were raiders along the Anglo/Scottish border from the 1200s to the 1700s. Here’s a set of free wargames rules for playing out these skirmishes.