The terms "open content" and "open educational resources" describe any copyrightable work (traditionally excluding software, which is described by other terms like "open source") that is licensed in a manner that provides users with free and perpetual permission to engage in the 5R activities:
This material was created by David Wiley and published freely under a Creative Commons Attribution 4.0 license at http://opencontent.org/definition/.
YSU Open Educational Resources
Affordable Learning Ohio
OhioLINK's series of initiatives to lower the cost of college for Ohio higher education students by assisting member libraries and campuses in identifying strategies for Open and Affordable Learning textbook and course material adoption, and helping locate statewide shareable library materials as well as open educational resources that are no cost or lower cost to students.
Creative Commons
Enables sharing and reuse of creativity and knowledge through the provision of free legal tools.
SPARC*
Global coalition committed to making Open the default for research and education.
40 Sources for Curated Educational Videos
American Institute of Mathematics Approved Textbooks
AMSER: Applied Math and Science Education Repository
Directory of Open Access Books
Internet Archive: Open Educational Resources
MERLOT: Multimedia Educational Resource for Learning & Online Teaching
National Academies Press: Sciences, Engineering, Medicine
PDXOpen: Open Access Textbooks
The Mason OER Metafinder searches 17 major OER platforms.