Brickhammer is a set of free wargames rules for playing wargames using Lego and minifigs.
Rules: General
Minifigz Lego Combat Rules
Minifigz is a set of rules for combat with Lego figurines.
Plastic Brigade
Plastic Brigade is a set of fun rules for dollar store toy soldiers.
Mike Mike Projectile Wargaming
Mike Mike Projectile Wargaming is a set of free wargames rules for games of that sort where you use some short of shooter (such as a rubber band gun) to knock over the figures.
Use with plastic only.
Flip It Paper Combat
From the authors:
Flipit: Paper Combat is a fast-paced tabletop miniatures game in a setting similar to WWII. Players lead forces from one of the four major nations into the conflict for Jurop. Armies are built of models players can download, print and build for almost free (you still have to supply the ink, paper, and glue). There are simple starter rules available, which may be supplimented by optional advanced rules to make gameplay more challenging
Hat Tip to Martyn for the link.