Tree of Battles is a set of free wargames rules for battles of the 14th to 15th Centuries.
medieval
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.
FUBAR Medieval Rules
Following up on their FUBAR modern rules, Gawd ‘elp Us Games offers FUBAR Medievals.
Ancient and Medieval Painting Guide
Here’s a nice — if not overly deep — painting guide to ancient and medieval armies.