Open Textbook Library
Run by the University of Minnesota, the Open Textbook Library is a collection of open textbooks, searchable by subject. Great interface, and the catalog is very good.
“Open textbooks are textbooks that have been funded, published, and licensed to be freely used, adapted, and distributed. 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.”