Bookboon.com - contains free textbooks, mainly business topics.
Boundless - textbooks based on free web content. Pay $19.99 to get flash cards, quizzes, and teach aids.
College Open Textbooks - provides training for instructors adopting open resources, peer reviews of open textbooks, and mentoring online professional networks that provide support to authors who open their resources.
Flat World Knowledge - the world's largest publisher of free and open college textbooks.
MIT OpenCourseware - open textbooks for science and engineering disciplines.
Noba - psychology textbooks.
OpenStax College - provides free college textbooks. Hosted by Rice University.
Saylor.org - provides free textbooks along with free courses.
TextbookRevolution - use it to locate free textbooks and courses.
University of Minnesota Open Academics - contains post-secondary textbooks.
WikiBooks - the open-content textbooks collection that anyone can edit.