Ramming Speed is a set of free wargames rules for righting naval battles with ancient galleys. The rules are designed for 15mm models.
Fleet of Battle is a set of free wargames rules for playing out battles between fleets of ancient galleys. Think Ben Hur. There also are several pages of free print-and-play galleys for use in lieu of lead miniatures. The rules require a knowledge of the commercial Field of Battle wargames rules.
Ray Lucas offers Capitana, a set of naval rules specifically for 16th Century galley warfare in the Mediterranean. They were written for the NLWG 2009 convention game.