Multidisciplinary repositories of open textbooks and similar resources. These repositories may also include multimedia or other courseware but they're primarily foused on open textbooks.

Collections of primarily public domain (out-of-copyright) books.

Open courseware could include textbooks, quzzies/tests, lectures, A/V materials, and other educational resources.

Other open textbook repositiory collections.

OER repositiories related to science, technology, and medical disciplines. 

A list of resources to help you locate open source images and educational videos.