Project Gutenberg: Offers over 42,000 free ebooks—choose among free epub books, free Kindle books, download them, or read them online.
https://www.gutenberg.org/

OpenStax: An educational initiative by Rice University providing free, high-quality, peer-reviewed college textbooks online and low-cost in print.
https://openstax.org/

Library of Congress Collections: Still and moving images, prints, maps, and other historical documents from the Library of Congress.
https://www.loc.gov/collections

Rashtriya Pustakalaya: A national digital library platform by the Government of India.
https://ndl.education.gov.in/home

National Academic Press (NAP Notes): Publishes a wide range of research-based academic content and resources.
https://notes.nap.edu/

UCL Press: University College London’s open access academic publishing platform.
https://uclpress.co.uk/

Librivox: Free public domain audiobooks read by volunteers from around the world.
https://librivox.org/

University of Michigan Press Ebooks: A selection of open access scholarly ebooks.
https://www.fulcrum.org/michigan