Gwenthia is a shared world fantasy roleplaying setting released under a Creative Commons licence. Now it is a core group that develop it, but under the licence anyone can use it for non commercial use. We are about 85% finished on a MRQ compatible ruleset for Gwenthia, which will be quite useable with BRP if you wish. Ok, plug over, here's the link:
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What I will say is that the process is very difficult, the balance between democracy, anarchism, shared ownership, egos, enthusiasm, apathy, friendship and the degree of benevolent dictatorship is very difficult to manage. It is a great thing and we have produced a great setting, but I would suggest that the way that that any project is set up is very very crucial. To that end I stand ready to answer questions and help with ideas.