These are some guides from a variety of institutions who have organized OER by subject or discipline.
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 79 different sources and contains 324,517 records.
OASIS is being developed at SUNY Geneseo's Milne Library.
The OER Metafinder performs a simultaneous search across different sources of open educational materials. From George Mason University.