Naval War is a set of free wargames rules for World War II naval battles. From the website:
Naval War is a Tabletop Naval Miniatures Game designed to be played on a table-sized battle area with World War Two ship miniatures. The game rules and any aids needed for play are free and available for download on the site. The game is not tied to a specific minatures line and is suited for many different game scales (1:1800 - 1:6000)
Naval War is a game, not a simulation. The aim of the game is to recreate the feeling and immersion you get when reading the battle reports of famous sea battle like the Battle of the Coral Sea or the battle of the Java Sea. The focus lies on the action and on the ships on the table, so a lot of the technical details have been worked into easy to read datacards and quick game mechanics.