Fleet of Battle is a set of free wargames rules for playing out battles between fleets of ancient galleys. Think Ben Hur. There also are several pages of free print-and-play galleys for use in lieu of lead miniatures. The rules require a knowledge of the commercial Field of Battle wargames rules.
Ironclad Draughts is a set of one-page rules for playing out battles between ironclads.
Hunters on the Shoreline is a set of free wargames rules for playing WWII coastal naval actions using 1/600 miniatures. It’s available from PT Dockyards.
The Jackson Gamers have a page with everything you need to know to start gaming Civil War Ironclads
Mini Nap is a set of Napoleonic Warfare rules for 6mm figures. The authors write:
MINI-NAP is a set of wargames rules specifically designed for playing a reasonably quick corps sized Napoleonic wargame using the 6 mm scale.
They grew out of my desire for a set of rules that would allow two players to play a traditional style, historically based Corps sized game (of around 35 to 50 units on each side) but one that would allow such a reasonably large game as this to be played and finished within about four hours (which they do).
Under the MINI-NAP rules, the basic force normally fielded is a Corps consisting of several Divisions. Unlike many other wargames systems, there is no points system involved in MINI-NAP. Players are encouraged to use an actual historical corps (and historical corps structures). That means you will need to do some research.
As a guide, the MINI-NAP Corps Guidelines give some general details on how many of the various nationalities involved in this period arranged their Corps (and also the Infantry Brigades, Cavalry Brigades, Mixed Brigades, Reserve Brigades and Reserve Artillery of the Divisions therein) and they also include how to rate units. Brigades are important within the game system.
Individual units manoeuvre, fire, fight hand-to-hand combat, etc, during the course of the game by the Brigade, as they did historically. Several Brigades can also manoeuvre by the Division if circumstances allow.