OER are teaching, learning, and research resources that reside in the public domain or have been released under an intellectual property license that permits their free use and re-purposing by others. ~ William & Flora Hewlett Foundation
For a more comprehensive overview on OER, see:
This is a sample of several OER Astronomy textbooks and course materials available. Additional OER in your subject may be found by searching the resources listed on the ACC Libraries primary Open Educational Resources Guide.