arXiv is a free distribution service and an open-access archive for scholarly articles in the fields of physics, mathematics, computer science, quantitative biology, quantitative finance, statistics, electrical engineering and systems science, and economics.
medRxiv (pronounced "med-archive") is a free online archive and distribution server for complete but unpublished manuscripts (preprints) in the medical, clinical, and related health sciences.
Another complementary archive to arXiv, bioRxiv is a preprint repository that where authors can share unpublished preprints in the life sciences (including biology, ecology, neuroscience and more).
OSF Preprints is a general preprint repository that covers architecture, business, engineering, life sciences, physical sciences and mathematics, arts and humanities, education, law, medicine and health sciences and social and behavioral sciences, among others.