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.
Steve Jackson’s Ogre is a classic game. I first got a copy when it was in a small bag with thin cardboard cutout counters. It’s gone through several incarnations since then, but the idea of hordes of GEVs and infantry against a single cybernetic tank. Now Dan Efran has a page describing how to build Ogre pieces with Lego. Its really cool, and I can’t wait until my kids outgrow the things and I inherit them.
Peter Guenther offers BrickMech, a
d6-based game where Mechs built of LEGO� bricks vie for ultimate control of the battlefield. Constructed in a unique modular system, Mechs can scavenge parts blown off other Mechs to improve their stats. The game is designed to be a rich gaming experience that’s simple to play.
Brickquest is a set of free miniature wargames rules for dungeon adventures along the lines of Heroquest or DungeonQuest. The catch here is that it uses Lego guys.