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
Rules: Medieval
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.
FUBAR Medieval Rules
Following up on their FUBAR modern rules, Gawd ‘elp Us Games offers FUBAR Medievals.
Late Medieval Skirmish Rules
The Late Medieval Skirmish Rules is a set of free wargames rules for large scale figures on an actual dining room table (with salt shakers, etc.)