CK-12 is a non-profit organization based in California founded with the mission to produce free and open source K-12 materials aligned to state curriculum standards and customized to meet student and teacher needs. Mostly STEM Science and Math textbooks.
Connexions is a dynamic digital educational ecosystem consisting of an educational content repository and a content management system optimized for the delivery of educational content. Connexions is one of the most popular open education sites in the world. Its more than 17,000 learning objects or modules in its repository and over 1000 collections (textbooks, journal articles, etc.)
Merlot – Multimedia Educational Resource for Learning and Online Teaching has Textbooks for Various disciplines.
Project Gutenburg offers over 38,000 free ebooks: choose among free epub books, free kindle books, download them or read them online.