SPARC (https://sparcopen.org/) describes OER as the foundation of Open Education: "open education encompasses resources, tools and practices that are free of legal, financial and technical barriers and can be fully used, shared and adapted in the digital environment." OER are teaching, learning, and research resources that are free of cost and access barriers, and also allow for legal permission for open use. Most OER include a Creative Commons license.
OER can have a major impact on the educational system, where the cost of textbooks and other educational resources would no longer be a barrier to education. More information about OER can be read in the article What is OER? Answers to 5 Questions About Open Educational Resources.
Here is a list of OER resources, focusing on textbooks.
In OER Commons, you can search, browse, and evaluate resources from a collection of over 50,000 high-quality OER, including: Full university courses, mini-lessons and simulations, open textbooks, and K-12 lesson plans and worksheets.
A public digital library of OER for higher education. On this site you can search collection, and use free software to create and collaborate to improve instruction through OER use.
A nonprofit based at Rice University, OpenStax provides access to open textbooks that are developed and peer-reviewed by educators. Textbooks primarily for AP and college classes in areas such as math, sciences, social sciences, and humanities.
Free and peer-reviewed textbooks, that are also openly-licensed to allow for reuse and alteration. A comprehensive reference site that points to open textbooks by a variety of authors and publishers.
Index of MIT OpenCourseWare and contains open textbooks or detailed course notes. Open materials on engineering, computer science, health sciences, mathematics, physics, and more.