Openly Available Sources Integrated Search (OASIS) is a search tool that aims to make the discovery of open content easier. OASIS currently searches open content from 98 different sources.
Open textbooks are free, online learning materials with Creative Commons licenses. There's some overlap among the collections linked below, but each has a particular focus and unique items as well.
OER repositories contain more than open textbooks. Learning materials in these collections include full courses, syllabi, images, presentations, videos, simulations, and many more.
The resources linked here include images and photos that can be downloaded for free with limited or no restrictions.