Stephan Hess has written Chariotmaster, a set of free wargames rules for playing a fantasy version of Roman chariot racing. What I like about these rules is that they don’t require you to play on a gridded board, like Circus Maximus and other chariot rules sets.
The good folks at Vis Bellica Online have this review of 15mm ancients wargames figure lines, including Essex Assyrians and Egyptians, Chariot Egyptians, Tin Soldier Greeks, Thracians and Persians, Xyston Greeks, Pass O’ The North, and Pendraken 10mm late Romans.
Acies is a very nice looking two player card game designed to simulate conflict between ancient armies. it looks like it would be a fun thing to play on off-nights when your regular miniatures games end early. All the parts you need are there to download and print out.
De Bellis Armatis is a set of free wargames rules for ancient warfare.
Corvus is a set of free wargames rules for fighting ancient sea battles.