Here are a set of Very Simple Generic Miniatures Rules.
Between 1968 and 1970, wargaming pioneer Charles Grant wrote a serialized wargames guide in Meccano Magazine. The serial is now available as a free download. This serial eventually became his classic Battle: Practical Wargaming.
Brickhammer is a set of free wargames rules for playing wargames using Lego and minifigs.
Minifigz is a set of rules for combat with Lego figurines.