(open source: available for use without permission or payment)
Connexions
"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."
Flatworld knowledge
A small but growing repository of free online textbooks.
Freeload Press
A repository of peer-reviewed, free e-books. Searchable by title or browsable by subject. Site contains advertising to pay for hosting the e-books.
Merlot
Peer-reviewed teaching and learning materials. A repository achieving national recognition in higher education.
Merlot open textbook collection - a growing subset of Merlot.
OER Commons
"A global teaching and learning network." Mostly K-12, but includes some college-level resources such as brief tutorials and powerpoints.
Purple math
Algebra help.
Teacher Tube
Educational videoclips, a YouTube for educators.
Textbook revolution
Student-compiled site in favor of instructors adopting free textbooks. Organized by broad category and searchable. Not all resources are complete textbooks.