A Hotter Fire is a set of free wargames rules for naval combat between civil war ironclads.
Rules: Naval
Away With Honour – Middle Ages Naval Warfare
Away with Honour is a set of free wargames rules for naval warfare in the Middle Ages by Tony Cullen.
Der Tag Naval Rules
Der Tag is a set of free wargames rules for WWI naval warfare in 1/6000 scale.
Ram or Die – Battle of Salamis Rules
Ram or Die is a set of free wargames rules (and paper figures!) for refighting the Battle of Salamis.
Seas of Lead WWII Naval Rules
Seas of Lead is a set of free wargames rules for World War II Naval games. Along with the rules are some good looking unit data cards that they made using the Magic Set Editor.