Periplus is a free two player wargame that abstracts a battle between two fleets of ancient greek warships.
Hoplon 3 is a nicely done set of ancients rules for large scale battles. Once commercially published, these now are available for free. The authors write:
The HOPLON rule system was designed from the start to use many of the troop types, mechanisms and army composition lists of DBM, while re-introducing those aspects of ancient warfare that have been lost through the pursuit of championship competitiveness. This allows players who wish to wargame utilizing realistic tactics, without having to sacrifice their competition-designed armies, to do so. HOPLON is by no means a DBM derivative in that it significantly differs in a number of crucial areas:
opponents move and fight �simultaneously� and can do so either organized into units or as unformed elements; various troop types may evade or break-off, all may flee or pursue, making the battlefield far more fluid and, at times, less predictable; shooting has received special attention and all troops that could do so may use this to their advantage, but purely missile troops may be limited in hand-to-hand combat; elite troops are truly capable when compared to untrained levies and can be devastating if
handled properly, and, although the PiP system is retained, better commanders have the ability to influence the mobility of their troops beyond the roll of a single die �at a price.
Pharaoh Rides To War is a set of rules for 20mm battles between Egyptians and Assyrians.
Pocket Civilization is a free, downloadable solo civilization building game that’s advertised as being compact enough to play on an airplane.
It looks very cool, and the Board Game Geek reviews are favorable.
Caveman Clash is a set of free wargames rules for playing skirmishes between cavemen. There also are paper figures if you don’t happen to have cavemen available.