Firepower is a set of free wargames rules for naval battles. The rules cover the period 1900 – 1945.
naval
A Hotter Fire ACW Naval Rules
A Hotter Fire is a set of free wargames rules for naval combat between civil war ironclads.
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.