eCampus Ontario Open Library is a great place to start. It offers over 180 Open textbooks from various disciplines.
BCcampus Open Textbook Project out of British Columbia currently lists more than 150 Open textbooks from twenty-four disciplines. Textbooks listed there can be used by anyone free-of-charge (digital versions) and instructors are free to make any modifications they wish to the text, as long a they attribute the source and, in turn, make available the revised work with an open license. Texts available through the site go through a peer review process.
OpenStax College is a nonprofit organization committed to improving student access to quality learning materials. Their free textbooks are developed and peer-reviewed by educators to ensure they are readable, accurate, and meet the scope and sequence requirements of your course.
Open Textbook Library is a tool to help instructors find affordable, quality textbook solutions. All textbooks in this library are complete and openly licensed.
INTECH is the largest Science, Technology & Medicine Open Access book publisher. Subjects include Health Sciences, Physical Sciences, Engineering and Technology, Physical Sciences, Life Sciences, Social Sciences, and Humanities.
Open SUNY Textbooks is an open access textbook publishing initiative established by State University of New York libraries. This pilot initiative publishes high-quality, cost-effective course resources by engaging faculty as authors and peer-reviewers, and libraries as publishing service and infrastructure.