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 71 different sources and contains hundreds of thousands of resources. (OASIS is developed at SUNY Geneseo's Milne Library).
From George Mason, the OER Metafinder helps find Open Educational Resources. The OER Metafinder launches a real-time, simultaneous search across 22 different sources of open educational materials.
Given the many “standards” of metadata in the OER universe, we can’t guarantee that every item retrieved is “Open” in the strictest interpretation of that term…so make it a practice to check the rights of any item you use.