Ross Macfarlane has a set of rules for privateer raiding actions during the American Revolution.
Men O’ War is a set of free wargames rules for Napoleonic sail. The rules are aimed at small actions, with one or two ships per player.
Battle War At Sea is a set of free wargames rules for modern naval warfare. It is designed for 1/6000 scale ships
Jutland: The Rules of the Game are a set of free wargames rules for large scale WWI Fleet Actions.
The Immortal Memory is a set of rule for refighting Trafalgar.