The BC Open Textbook Project was tasked with creating open textbooks for the "top 40 highest-enrolled subject areas." You can find, adopt, and modify any textbook in the collection. These text books have been adopted over 500 times already....
Affiliation: British Columbia Ministry of Education, BCcampus

Pairs open source college and AP textbooks with adaptive learning technology.
Affiliation: Rice University

"These books have been reviewed by faculty from a variety of colleges and universities to assess their quality. These books 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."
Affiliation: University of Minnesota. Supported by the Center for Open Education and the Open Textbook Network

Milne Library Publishing at SUNY Geneseo manages and maintains Milne Open Textbooks, a catalog of open textbooks authored and peer-reviewed by SUNY faculty and staff.