When you begin your search for OER applicable to your course, there are several categories of resource repositories.
These major databases aggregate OER content across disciplines and audiences. They have search interfaces by subject and oftentimes education level. In most of them, you'll find reviews by other instructors who have used the materials in their courses, sometimes in a 1-to-5-star ranking. It can take some tweaking of your searches and time to assess results, but these repositories are the best place to start looking for OER.
If you're looking to use an entirely open textbook for you course—rather than other, more individualized OER materials—there are several places where you can search for full textbooks that are reviewed by users.
Duke also has a collection of full courses made openly available online through Coursera.
OER Commons
A large repository that includes a wide range of OER. Use the search limiters to narrow by education level, material type (i.e. Textbooks, Lessons, etc) and subject.
OASIS
Openly Available Sources Integrated Search (OASIS) search tool for finding open content, including textbooks, courses, course modules, audiobooks, and video from SUNY Geneseo.
OPEN Washington OER Resource Repository
A collection of open images, textbooks, course materials, videos, and audio sources curated by OPEN Washington, a network existing to educate teachers about Open Educational Resources.
Merlot
A long-lived online teaching, learning, and faculty development service contributed to and used by an international education community. Includes open textbooks. Material Type and Audience limiters in Advanced Search can be helpful.
Mason OER Metafinder
Launches a real-time, simultaneous search across 22 different sources of open educational materials rather than one database.
Over four million peer-reviewed, free scholarly articles on a variety of academic subjects including business, engineering, social sciences, law, and humanities.
Open Textbook Library
Complete textbooks that have been reviewed by a variety of college and university faculty, and can be downloaded for no cost, or printed at low cost. All textbooks are either used at multiple higher education institutions; or affiliated with an institution, scholarly society, or professional organization.
OpenStax
Nonprofit based at Rice University that has published more than 20 open source, peer-reviewed, textbooks in common undergraduate subjects like Algebra, History, and Psychology. Note that OpenStax textbooks can also be found through a search of the Open Textbook Library (linked above).
Directory of Open Access Books
Not limited to textbooks, the DOAB provides a searchable interface for finding openly available academic titles across the web.
Creative Commons Search
Creative Commons has created this really useful search engine that lets you search for resources that are CC licensed. The image search is the most useful.
Wikimedia Commons
Historical and contemporary images contributed by participants. Millions of high-resolution images of art, architecture, design, people, historical events, diagrams, maps, and more.
Flickr Creative Commons
Images provided by Flickr users that are available under a Creative Commons license.
Getty Open Content Search
Search for open content across the Getty's collections, including the J. Paul Getty Museum, the Getty Conservation Institute, the Getty Foundation, and the Getty Research Institute.
Open access images from many of the Smithsonian Museums.
Coursera at Duke
Openly available full online courses designed and taught by Duke instructors.
