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/

Open Access E-books: A growing collection of contemporary open access e-books covering a wide range of subjects including history, music, poetry, technology, and fiction.
https://www.loc.gov/collections/open-access-books/about-this-collection/

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Library of Congress Collections: Still and moving images, prints, maps, and other historical documents from the Library of Congress.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

Wiley Online Library (COVID-19 Resources): Free access to research related to the novel coronavirus.
https://novel-coronavirus.onlinelibrary.wiley.com/

Cambridge University Press - Open Access: Open access research and publications by Cambridge University Press.
https://www.cambridge.org/core/publications/open-access

Hindawi (via Wiley): Open access journals across a variety of scientific disciplines.
https://onlinelibrary.wiley.com/action/showPublications?PubType=journal

Oxford Open Access Journals: Open research and open access journals from Oxford University Press.
https://academic.oup.com/pages/open-research/open-access

Taylor & Francis Open Access Journals: Access to open research publications in multiple disciplines.
https://www.tandfonline.com/openaccess/openjournals