Following up on their FUBAR modern rules, Gawd ‘elp Us Games offers FUBAR Medievals.
Here’s a nice — if not overly deep — painting guide to ancient and medieval armies.
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.)
Al Hewitt and Ric Willey offer Jousting, a set of free wargames rules for medieval tournaments.
Argad is a set of free wargames rules for skirmishes in the dark ages and for pirates in the 17th and 18th centuries.