OASIS (Openly Available Sources Integrated Search) is a search tool that aims to make the discovery of open content easier. OASIS currently searches open content from 64 different sources and contains over 160,000 records.
Open textbooks come in a variety of downloadable formats (PDF, ePub, .xml, .docx) and can be printed and bound inexpensively for students preferring a hard copy. The open licenses attached to these books give faculty the freedom to pick and choose the parts of a book(s) that they feel will best help students achieve the course learning outcomes.
BC Open Textbook Project: Run by the University of British Columbia, and has open textbooks organized by discipline.
College Open Textbooks: Aggregation site with a focus on trying to develop better peer review of OERs. Search by subject, reviews and accessibility. Funded by The William and Flora Hewlett Foundation.
Directory of Open Access Books: Thousands of books from hundreds of publishers.
Libre Texts Chemistry: Includes course textbooks and supplemental learning resources. Supported by the National Science Foundation and the US Department of Education's Open Textbook Pilot Program.
Merlot: Multimedia Educational Resource for Learning and Online Teaching. Tens of thousands of learning modules, tutorials, readings, videos, textbooks, etc., all available (check licensing) for educational use.
OER Commons: Repository of OER materials: textbooks, activities, video, images, case studies, etc. Browse by Subject/Discipline or Material Type.
Open Textbook Library: Run by the Open Textbook Network, an organization of more than 600 higher education institutions. Content is not centrally vetted, and relies on peer review. More than half the content has been reviewed.
OpenStax: Peer-reviewed, openly licensed.