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.
Role Playing 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?
Men and Supermen Super Hero Role Playing Game
The Men and Supermen Role Playing Game is designed to allow super powered characters to fight along side normal characters. In doing this the RPG takes on what I think is one of the most difficult of tasks — creating a common scale between units of disparate strength.
Under The Broken Moon Role Playing Game
Under the Broken Moon is an RPG based on the Thundarr the Barbarian cartoon series. The game uses a mechanic where you roll a number of dice equal to your trait score and sum the results.
Death or Glory Fantasy Role Playing Game
Death or Glory uses a percentile dice system and has medieval, magic and firearms supplements. In addition, there are rules for a medieval board game that allows you to lay out larger battles.