Wrath of the Gods is a set of free wargames rules for skirmishes with a classical mythological flavor.
Justified Ancients is a set of free wargames rules for simple ancients battle games. The author writes:
Justified Ancients is a fast paced element based wargame that takes simple mechanics to produce a beleivable result. Easy to learn they are percet for filling in that last half hour before the Pub or playing a huge battle in two hours.
Wargames Research Group has a page with lots of their older sets available, including the Fifth Edition Ancients, which were the standard for many years.
The Games: Blood and Spectacle are a set of free wargames rules for gladiator games.
Charles and David Sweet were among the founders of American wargaming. Here are his rules for ancient wargaming.