Each of the open textbook collections listed below have been peer-reviewed by faculty in that particular discipline to ensure quality in keeping with current academic standards:
Open Stax College Textbooks from Rice University
Open Stax has a growing collection of peer-reviewed, openly licensed university textbooks in a variety of disciplines, from the sciences to the social sciences to the humanities. Faculty wishing to adopt a textbook must register for a free account.
Open Textbook Library from the University of Minnesota
The Open Textbook Library has a collection of over 100 peer-reviewed and openly licensed titles in a wide range of disciplines. It is supported by the Open Textbook Network, a collaboration of library partners from colleges and universities across the country.
These resources have not necessarily been peer-reviewed but can still serve as a great sources of quality OER for course adoption, with or without modification: