The Open Textbook movement focuses on the creation of books that are built specifically for use as free or low-cost options for education. Many of the collections will have links to the same books, but each repository has a particular focus, and items you can't find in other collections. This is not a complete list, but will be updated periodically.
The OpenCourseWare (OCW) movement began in the United States with the Massachusetts Institute of Technology in 2002. Open CourseWare Materials can include syllabi, written lectures, assignments, readings, videotaped lectures, and audio lectures. This is not a complete list, but will be updated periodically.
Collections of images that are either openly accessible or searchable by open license.
Using Google to Search for OER