Open Text Book is a registry of textbooks (and related materials) which are open — that is free for anyone to use, reuse and redistribute. It is run by the Open Knowledge Foundation. http://www.opentextbook.org/
The California Open Source Textbook Project (COSTP) is a collaborative, public/private undertaking. It has been created to address the high cost, content range, and consistent shortages of K-12 textbooks in California. http://www.opensourcetext.org/
Connexions is a place to view and share educational material made of small knowledge chunks called modules that can be organized as courses, books, reports, etc. Anyone may view or contribute: http://cnx.org/
The Online Content Repository (OnCoRe) Blueprint Project presents a sustainable national model for the creation of statewide digital repositories. Supported by a grant from the Fund for the Improvement of Postsecondary Education (FIPSE), the OnCoRe Blueprint Project provides state and other initiatives with a “how to” manual, resources, and research background for developing a standards-based educational repository. OnCeRe Report view here