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.
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?
Ars Magica is a role playing game set in Mythic Europe — nominally 1220 AD, but a land inhabited by faeries, daemon and other magical creatures. Its a great game that I’ve enjoyed playing over the years.
Ars Magica set the benchmark for magic in fantasy roleplaying. It pioneered the storytelling style of roleplaying that has become so popular today. Its setting, Mythic Europe, sparked the imaginations of fantasy fans and history enthusiasts alike. The 272-page fourth edition introduced improved systems in several key areas such as combat, character advancement, and covenant generation. This version of the game retained and improved upon Ars Magica’s powerful and flexible magic system – widely regarded as the best rules for magic in all of gaming.
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 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.