GMT Games offers a free download of a Battle of Agincourt game. If you’re familiar with their commercial board war games, you’ll know that they have wonderful graphics and gameplay, both of which are on display here.
Miniatures Rules
Battle of the Little Big Horn Rules
Battle of the Little Big Horn is a set of free wargames rules for fighting Custer’s Last Stand. You need to turn over your email to get a copy of the rules, but it doesn’t look to me like some sort of sophisticated anti-wargamer phishing scheme.
Dark World War I Rules
Dark World War is a set of free wargames rules — actually pay what you want — for playing WWI games that include demonic and mythological creatures.
Battle of Jutland Quick Play Rules
Matt Fritz has written a set of quick play rules called Day of the Dreadnaughts for recreating the Battle of Jutland. The set includes links to paper models, in case you want to try it, but don’t hvae the appropriate miniatures.
Design and Fight A Civil War Ironclad
Pete Pellegrino has written a set of free miniature wargames rules called Damn The Torpedos. In the game, you customize your ironclad, and then take it out to fight in the Cvil War battle of Mobile Bay.