Project Gutenberg (Mythology, Legends and Folklore. Full text of many world mythologies.)
Internet Sacred Text Archive (Full text of many world mythologies and legends.)
Te Ara Encyclopedia of New Zealand (Maori)
National Museum of the American Indian (do a collections search for origin stories)
Inuit Traditional Stories (Encyclopedia of Canada)
Perseus Digital Library (Tufts University. Greco-Roman)
Theoi.com (Greco-Roman)
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A general encyclopedia. Useful for quick background knowledge about a topic.
Database with guidance for novice researchers. (Grades 6+)