These databases are a good starting point to find a wide variety of open educational resources.
Although not as well-known as the above databases, these sites can also be good places to find open educational resources, including textbooks and other material.
Open educational resources don't have to be just textbooks; any material you use in the classroom can be open. These sites focus on other materials - images, videos, and audio - that are free to use and, in some cases, licensed for adaption.