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This is a list maintained at the Wynwerod RPG forum. Licenses include CC0 / Public Domain, CC-B, CC-BY-SA, CC-BY-NC, CC-BY-NC-SA, OGL 1.0a and others. 

If the forum is down, here's a wayback machine link: https://web.archive.org/web/20240105063728/https://forum.wynwerod.com/t/the-great-list-of-open-license-rpgs/154