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
"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
This site features crowd-sourced open texts created by scholars. They also offer institutional customization with quizzes and analytics as a premium service. ($29.99 per student per course for lifetime access)