Der Tag is a set of free wargames rules for WWI naval warfare in 1/6000 scale.
naval
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.
Destroyer Commander
Destroyer Commander is a variant for Mongoose Games’ Victory At Sea that focuses on smaller ship actions. I have always wanted to do Destroyer and PT Boat games.
Index Card Ships
There are no actual instructions here, but The Penny Whistle has some really neat photos of index card ship models. They should be relatively easy to replicate and look like they could be used in a fun game.