Touhou
This game series has been very famous for being a large source of doujin content as different types of media such as songs, artworks, games and stories have been made by the followers of Touhou Project, and as a result it has gained a very large following outside Japan, due to the small amount of rules regarding copyright set up by ZUN. These works are usually sold at Fan conventions such as Comiket, which is notable for highest doujin circle participation, and the official Reitaisai, where early versions of upcoming games are put on display there.