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 75 different sources and contains 176,005 records.
The Mason OER Metafinder allows you to simultaneously search across the multiple OER repositories (e.g., MERLOT II, OAOpen.org, OER Commons, OpenStax CNX, etc.) as well as many open archival repositories (e.g. HathiTrust, Internet Archive, Project Gutenberg, etc.).
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.