Trireme, from Andy Watkins, is a set of free naval wargames rules. These are of the one-page variety, which means that there is much to recommend them. I also like the staggered movement system based on boat speed, which reminds me of the old Car Wars system.
galley
Ramming Speed 15mm Galley Rules
Ramming Speed is a set of free wargames rules for righting naval battles with ancient galleys. The rules are designed for 15mm models.