"Open Educational Resources (OERs) are any type of educational materials that are in the public domain or introduced with an open licence. The nature of these open materials means that anyone can legally and freely copy, use, adapt and re-share them. OERs range from textbooks to curricula, syllabi, lecture notes, assignments, tests, projects, audio, video and animation." (UNESCO)
Adopting OERs for your classes is a great way to reduce textbook costs for your students.