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.
The Junior General offers a set of rules for the English Civil War battle of Marston Moor.
Ryan Scow offers a set of free medieval siege rules.
In The Year of One Reed: Final Conquest of the Aztecs is a set of fast play rules for students about the Spanish conquest of Tenochtitlan.