De Bellis Velitum is a set of free wargames rules from the legendary Phil Barker that features:
two small forces, each consisting of 1 to 4 sub-units, each in turn of 4 to 12 singly based 25mm figures, totalling up to 24 army points (AP) and controlled by a single player. Figures vary in cost between 6 AP and 2 AP. One figure of the most expensive mounted type or most expensive foot type present is nominated as the force’s commander and one figure of each group is nominated as its leader.
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.
The I Live With Cats Blog has a variant for Lion Rampant featuring Late Romans.
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.