MIT OpenCourseWare: Some of these online textbooks are open-licensed electronic versions of print books. Others are self-published online books, or course notes which are so thorough that they serve as an alternative to a conventional textbook.
MERLOT: MERLOT (Multimedia Education Resource for Learning and Online Teaching) is a free and open peer reviewed collection of online teaching and learning materials and faculty-developed services contributed and used by an international education community.
Open Course Library (OCL): OCL materials were created through Washington State Board for Community & Technical Colleges grant, but are freely shared to the world with a Creative Commons Attribution-only license (CC-BY).
Open Educational Resources Commons: OER Commons is a growing collection of teaching, learning and assessing materials created by individuals or organizations who retains few, if any, ownership rights.
Open Education Consortium: The Open Education Consortium is a non-profit organization that supports openness, collaboration, innovation and free distribution of educational materials.
Open Oregon Educational Resources: Promotes textbook affordability for community college and university students, and facilitates widespread adoption of open, low-cost, high-quality materials.
OpenStax: Rice University’s initiative provides free, peer-reviewed textbooks that are written by experts. You can read the textbooks online, download a full-text PDF or order the low cost ($20-$40) print version.
Open SUNY Textbooks: Established by State University of New York libraries, this initiative publishes high-quality, peer-reviewed textbooks available to educational institutions around the world.
Open Textbook Network (OTN): Hosted by the College of Education and Human Development at the University of Minnesota, OTN provides a growing catalog of free, peer-reviewed, and openly-licensed textbooks.
Textbook Revolution: Textbook Revolution is a student-run site dedicated to increasing the use of free educational materials by teachers and professors. On this site you'll find links and reviews of textbooks and select educational resources. Some of the books are PDF files, others are viewable online as e-books, or some are simply web sites containing course or multimedia content.
Wikibook: The Wikibooks project was started in 2003, and has since become one of the largest free e-book websites in the world.