The Open Textbook movement focuses on the creation of books that are built specifically for use as free or low-cost options for education.
These may include textbooks, but include other open educational resources such as tutorials, videos, lesson plans, and assignments.
OER Commons
"OER Commons is a dynamic digital content hub, offering a suite of OER implementation supports."
Open Educational Resources: Resource Roundup, from Edutopia
Updated posting, a great guide to resources for getting started, organized with many links!
iLumina
A digital library of sharable undergraduate teaching materials for chemistry, biology, physics, mathematics, and computer science. Created by the National Science Digital Library, iLumina is designed to quickly and accurately connect users with the educational resources they need. These resources range in type from highly granular objects such as individual images and video clips to entire courses.
Discipline-Specific Sources, Introduction to OER - openstax cnx
Page identifying different OERs, mostly instructional units or objects, within Rice University's openstax course on OER.