Assault on Innsmouth is a set of free wargames rules for involving the Lovecraft Mythos. The players are government forces taking on the horrors in Innsmouth. The forces of the mythos are run by the system. The rules are a heavily modified version of the Two Hour Wargames system. There are also expansions at Board Game Geek.
Miniatures Rules
Coal and the Kaiser naval miniatures rules
Coal and The Kaiser is a set of free naval miniatures rules.
Author Leonard Heinz writes:
This is a game of low to moderate complexity portraying tactical naval combat in the period from 1904 through 1918. While it will not yet you play Jutland in a day, it will let you play some fairly large actions reasonably quickly. It uses a scale of 4 minutes to the game turn and 1000 yards to an inch on the playing surface. The rules focus on big ships and big guns: battleships (and particularly the Dreadnought-type battleships that began service in 1905), battlecruisers and armored cruisers.
A l’Ombre des Aigles Napoleonic Miniatures Rules
Dan Brown offers A l’Ombe des Aigles, a set of free miniatures wargaming rules for Napoleonics miniatures.
Warhammer 40K Squat Codex
I stopped buying Warhammer 40K stuff when they dropped the squats as a major race. In my basement workshop, I’ve got a horde of the stunties, in regular battle gear, on trikes, in exo-armor … you name it. You can find an online squat codex in the War Vault.
The Natives Are Restless Tonight
Matakishi has written The Natives Are Restless Tonight, a set of free wargaming siege rules for any game involving a large band of natives, and a small, but stalwart group of westerners. They would work for any game in the colonial era, including the Zulu War, Boxer Rebellion, Foreign Legion, North West Frontier and Indian Mutiny.
The rules use a card activation system, and involve rolling handfulls d6s. Casualties are determined by summing the attack dice and removing a figure for every multiple of 6, 9 or 12, depending upon the defensive situation.