Snapshot is a set of free wargames rules for playing modern submarine games using 1/700 ships.
Here are a set of WWII Naval rules created by the author for playing with his homemade WWII ships.
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!).
De Bellis Navalibis is a set of free wargames rules for playing ancients naval wargames.
By Jeff Peach & Les Booth offer a set of free wargames rules for ACW River and Coastal Warfare