Cthulhu: The Great Old One is a simple, free print-and-play card game.
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Arkham Investigator Paragraph Game
Arkham Investigator is a free paragraph based game that uses many of the same mechanics as the old Sherlock Holmes: Consulting detective game.
Dice of Arkham Print and Play Game
I’m a big fan of all things Lovecraft, so this one has my attention: Dice of Arkham is a solo, print-and-play game of Elder Gods and the Things man Was Not Meant To Know.
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Hysterical.
Strange Aeons 18th Century Modifications
Strange Aeons is a commerical rules set, but if you already own it, you may be interested in these free modifications for playing in an 18th Century setting.