Ian Hopping offers My Kingdom For A Horse, a set of free wargames skirmish rules for the dark ages and medieval periods.
Andy Clipsom offers a set of rules called Free Company, which is for games set in the longbow era. From the author’s introduction:
I started work on these rules to fulfil some personal gaming criteria. I wanted rules for a small battle but not a skirmish. The command level would therefore be around the unit, rather than the division. I imagine medieval unit commanders using their initiative and experience within an overall game plan, so the player is being a succession of unit commanders with limited initiative, plus the big cheese, who has slightly more control over those around him.
My figures are 25-28mm, but I have only a small space to game, so distances reflect this. I also like multiple bases for ease of movement, so I have returned to simple casualty rosters (though you could use markers). The system should work easily enough for multiple bases in other scales, as it is based on a nominal strength of the base rather than an actual headcount of figures.
The game assumes a game scenario agreed in advance, rather than an equal points, random set up game. It is assumed things not covered in the rules (like scouting, ambushes, destruction aof property) will be covered in the scenario.
Players won�t find a lot of innovative new mechanisms and while some influences are obvious (e.g. the use of DBA PIP-style mechanism to limit the number of actions a player can take), others are lost in the mists of time.
Junior General has a simple set of Medieval Castle siege rules. The site also has links to appropriate paper figures, castle models and siege equipment.