Preprint repositories
bioRxiv (pronounced "bio-archive") is a free online archive and distribution service for unpublished preprints in the life sciences (articles in bioRviv are not peer-reviewed, edited, or typeset before being posted online).
medRxiv (pronounced "med-archive") is a free online archive and distribution server for complete but unpublished manuscripts (preprints) in the medical, clinical, and related health sciences.
Open access
CORE is a not-for profit organization that aggregates open access research papers from around the world, while using unique APIs to allow access to content and data - which is useful for text mining.
DOAB is a community-driven discovery service that indexes and provides access to scholarly, peer-reviewed open access books and helps users to find trusted open access book publishers.
DOAJ is a community-curated online directory that indexes and provides access to high quality, open access, peer-reviewed journals (you can search not only for entire journals but also articles within those journals).
Direct to Open publishes 80-90 new MIT Press scholarly monographs/edited collections titles per year, while providing participating libraries with term access to backlist/archives (~2,500 titles), which would otherwise be gated.
Open education
OER Commons is a public digital library of open educational resources - explore, create, and collaborate with educators around the world to improve your curriculum.
Open Textbook Library offers 1449 open textbooks all licensed by authors and publishers to be freely used and adapted (download, edit, and distribute them at no cost).
Pressbooks Directory is a free, searchable catalogue that includes 6,558 open access books published by 176 organizations and networks - it's easy to copy, revise, remix, and redistribute any openly licensed content found here.
MIT OpenCourseWare is a free and open collection of material from thousands of MIT courses, covering the entire MIT curriculum (download files, send to friends/colleagues, modify, remix, and reuse - just remember to cite OCW as the source.