Games Workshop’s Lord of the Rings game is a marvelous skirmish system that is readily convertable to many other periods. Hey — Games Workshop proved it by releasing Legends of the Old West, a set of cowboy skirmish rules.
Voltigeur is a conversion of the system for Napoleonic gaming.
I wish I had some 25mm Napoleonic figures so I could try it.
Mike Fischer offers a set of miniature rules called To A Pulp (nice pun). He writes
Pulp gaming, in which each player controls a character or faction from a 1930’s pulp thriller, is growing in popularity. Such games combine the visual appeal of miniatures gaming with aspects of role-playing games, allowing the players to control the actions of tough private eyes, seemingly-helpless dames, brave explorers, and all the rest. I wrote these rules mostly for my own amusement.
The rules are made for simplicity and fast play. The game is meant for use with miniatures, but you can download a set of paper minis for free (see Section XI). All you have to provide is a few six-sided (d6) and ten-sided (d10) dice. The game is meant for two or more players, but it can be used for solo gaming as well.
Ryan Scow offers a set of free medieval siege 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 is a set of free rules for playing science fiction, fantasy or historical gladiator games.