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 OER material at sites like HathiTrust, DPLA, the Internet Archive, and Project Gutenberg.
The 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 80 different sources and contains 331,461 records. OASIS is being developed at SUNY Geneseo's Milne Library.
The Open Textbook movement focuses on the creation of books that are built specifically for use as free or low-cost options for education. Many of the collections will have links to the same books, but each repository has a particular focus, and items you can't find in other collections. This is not a complete list, but will be updated periodically.
The OpenCourseWare (OCW) movement began in the United States with the Massachusetts Institute of Technology in 2002. Open CourseWare Materials can include syllabi, written lectures, assignments, readings, videotaped lectures, and audio lectures. This is not a complete list, but will be updated periodically.
Using Google to Search for OER