The downloadable D20 system reference documents provide most of what you need to play any of the d20 system games that stem from the original Dungeons and Dragons system.
The Miniature Wargaming Editor
Afternova Print and Play Game
Afternova is a free print-and-play game about collecting resources from a solar system after it has been destroyed by a supernova.
Battle of Salamis Rules
Matt Fritz offers a simple set of rules for recreating the 450 BC naval battle of Salamis between the Greeks and the Persians. The site even has links to paper models that you can use to play the game.
Brutal Hack and Slash Role Playing Game
Brutal is a basic hack and slash RPG that uses a dice pool system with opposed rolls to resolve an action. The game is designed to handle massive battles quickly. Author David Stanley writes:
This game is for serious players only! Hordes of Heart-stopping-pounding, Heavy-hitting, Hack-and-Slash action adventure await! This game is not for sissy players who don’t want their characters to die. In a BRUTAL game, your characters get killed all the time. How long can your characters stay alive?
Maeshowe Solo Print and Play Game
Maeshowe is a solo print and play game about tomb robbers in AD1150. From the game’s description:
According to the Orkneyinga Saga, Earl Rognvald Kali Kolsson broke into the chambered tomb of Maeshowe, on Orkney, in AD1150. In AD1153, Earl Harald Maddadson and a group of his men sheltered there during a snowstom. The sagas tell us that Rognvald found treasure there and that two of Harald’s men went mad. When the Victorians opened the tomb in 1861, they found Runic graffiti demonstrating that both of these episodes were based on real tomb-breaks. This game combines the two events from the 1150’s and casts you as Harald and Rognvald.
Th game uses standard playing cards (or you can print specialized cards) a board and a few tokens.