This is a big, huge, massive list of known places to look for OER because, sadly, there's no one place to look for OER. It's intended for librarians assisting faculty in finding OER for courses, but anyone can take a look. Not comprehensive!
A repository of links to freely available texts that replace a traditional reader for Composition courses, from the library at Columbus State Community College (Ohio).
Stand-along teaching or learning modules covering health professions education, provided by the Association of American Medical Colleges in partnership with the American Dental Education Association and the American Psychological Association.
Free, open source, online course management system for mathematics and other quantitative fields. Students can do practice sessions on their own, or faculty can integrate the textbooks and exercises into a course.
"All textbooks are either used at multiple higher educational institutions, or affiliated with an institution, scholarly society, or professional organization." Maintained by the University of Minnesota.