Here’s a fun looking set of medieval skirmish rules called Bashing the Bishop. The title tells you just how serious they are.
Tree of Battles is a set of free wargames rules for battles of the 14th to 15th Centuries.
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! is a set of free games rules for medieval jousting.
Following up on their FUBAR modern rules, Gawd ‘elp Us Games offers FUBAR Medievals.