Here is a simple, two page (front and back) set of rules for the 100 Years War by Eric Wood.
Chevauchee is a set of free wargames rules for man-to-man combat during the 100 Years War.
Vexillia has a selection of free flags for various European armies from the 14th, 15th and 16th centuries. These include 100 Years War, Wars of the Roses, as well as Spanish, Swiss, Landsnechts, Savoyed, Venetian and Imperialist.
Look in the right hand sidebar on the page for the links.
Eric Wood offers a set of free wargames rules for playing the 100 Years War battle of Crecy
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.