Supported by the Center for Open Education and the Open Textbook Network, the Open Textbook Library is a collection of OER. You can browse by subject or search the collection more broadly.
Available through Rice University, OpenStax is a collection of open textbooks on a set list of subjects. These books can be viewed on the web or downloaded as PDFs. If you let OpenStax know you are using their textbook you will be provided with a link to include in your syllabus/D2L page.
Created by the Institute for the Study of Knowledge Management in Education, OER Commons is a collection of OER. You can search the collection and focus by subject, education level, or educational standards.
Provided through the California State University system, MERLOT is a searchable collection of OER. Searches can be filtered by content, material type, or peer review status, among other things.