Richard Bradley’s Medieval Skirmish rules are offered for free on the Tyneside Wargames Club site. There actually are quite a few free wargames rules there. All look quite playable and fun.
designed for fairly large-scale skirmishes; up to about 80 men a player. They will do for many periods or settings, so long as troops can be classed into three distinct broad categories of quality. The rules emphasise the importance of sticking together for mutual protection and the value of weapons with longer reach: spearmen can fight in two ranks, pikemen in three. Men in the second rank with javelins can support.
The rules recognise two ways in which figures may be combined, Contingents and Groups. Contingents are real units (e.g. a Contingent of 15 archers from Berwick under their bill-armed vintnar) and are used for organisational and morale purposes in these rules. A good size for a contingent is 32 points. Groups are temporary (sometimes fleeting) arrangements of men occurring in combat situations. Hand-to-hand combat is fought between Groups (which may be as small as one man each) and casualties occur almost at random within the Group
Also, an add-on for leaders and dueling.
Stout Hearts is a set of free wargames rules inspired by the classic Hal Foster Prince Valiant comics.
Men at Arms is a set of one-brain-cell rules from the fertile brain of Jim Wallman. Jim writes:
These are rules for playing a wargame with toy soldiers. It is intended for several players – say 4 or more. Players control significant leaders – the key lords or knights, who in turn have contingents of fighters under them. Why �One Brain Cell�? Well, many sets of wargame rules these days are horrendously complicated, with big thick rule books to read, dozens of additional books to get (at unreasonable expense) and exceptionally complicated rule mechanisms that take ages to work out. My brain is too simple for this, so I tend to write rules that one require a single brain cell to use and understand. This tends to make games easy to learn and play, and, amazingly, are just as much fun as the dense and complicated game rules for which you have to pay a King�s ransom. Odd, isn�t it?
Simplus Medieval Miniaturis is a set of free wargames rules for simple medieval miniatures battles.