Open Education Resources (OER) are "teaching, learning, and research materials that are free for people everywhere to use and repurpose" (https://hewlett.org/strategy/open-educational-resources/).
A number of UM-Flint classes, primarily in the mathematics department, have adopted OER textbooks from OpenStax.org, a non-profit organization affiliated with Rice University. OpenStax OER textbooks are unique; in addition to the freely available textbooks online they also sell affordable print versions for students then still want a physical book.
This guide includes a list of courses that use OER textbooks along with the links to the books.
The University of Pittsburgh Library compiled a fairly comprehensive OER listing here https://pitt.libguides.com/openeducation/biglist.
The following half hour webinar is a good introduction to open textbooks and open educational resources. Our colleagues at Wayne State recorded this during OER Week 2020.