Ryan Scow offers a set of free medieval siege rules.
rules
Republic and Empire Rules
Arofan Gregory offers Republic and Empire, a set of free wargames rules for 19th Century warfare.
Gregory writes:
These rules are intended to be used for any land conflict from the Napoleonic Wars through the Second Anglo-Boer War. Encompassing both colonial actiosn and conflicts between technologically advanced powers, they are intended for those who wish to recreate the lesser known conflicts of the period, for which specifically tailored rules sets are often not available. They are designed as a framework that is easily modified to fit the specific situation being recreated. Alternately, these rules are good for those who wish to play battalion-level games that move quickly, as a change of pace from more complicated games … Toward this end, any consistent basing system can be used …
Roar of the Crowd Gladiator Rules
Roar of the Crowd is a set of free rules for playing science fiction, fantasy or historical gladiator games.
They Don’t Like it Up ‘Em Colonial Campaign Rules
They Don’t Like it Up ‘Em is a set of free wargames rules for colonial campaigns.
.45 Adventures Free Downloads
Rattrap Productions has a set of free downloadable demo rules for its .45 Adventure miniatures rules. There also are a number of free scenarios.