"Open educational resources (OER) are any resources available at little or no cost that can be used for teaching, learning, or research. . . such materials are generally released under a Creative Commons or similar license that supports open or nearly open use of the content." (Educause, 2010)
Take advantage of Google's Advanced Search to limit by results. Select the appropriate permissions under "usage rights"
Use the Mason Metafinder to search several OER repositories simultaneously.
The searches seventeen targets in real-time, instantly returning the top several hundred or so most relevant hits from each site. Because it is a real-time search, it can take a bit longer than searchers of pre-indexed, curated content; however, as compensation the results returned are absolutely for each search target.
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 57 different sources and contains 159,459 records.