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. Many of the collections will have links to the same books, but each will have a particular focus, and items you can't find in other collections.
OER repositories contain more than just open textbooks. Learning materials in these collections include full courses, syllabi, images, presentations, videos, simulations, and many more.