Heart of Oak, Skin of Iron is a set of free wargame rules for naval combat in the years 1852 – 1870.
A Hotter Fire is a set of free wargames rules for naval comabt between civil war ironclads.
From the Naval Wargames Association comes Ironclad, a set of free wargames rules for ACW ironclads.
Steam Iron and Tin is a set of 15mm ACW naval rules that was distributed to people who attended the 1985 Origins Convention in Baltimore (I was at that convention!).