(Possibly) the world's only open-source RPG, Dominon was created by Dominion Games and by role-playing gamers the world around.

It's free as in beer and also free as in speech, because you can both download the complete rules from their website and submit changes and suggestions which can be incorporated into their official ruleset.

The Dominion Rules are based exclusively on a d12. It is a high fantasy role-playing environment, in the style of Dungeons and Dragons, but without the TSR/WotC/Hasbro monopoly to get in the way of pure gaming fun.