Below are some of the most comprehensive OER collections available. There are many subject specific resources available online as well, and often a simple search will steer you in the right direction. Most of the sources below offer options for a variety of subjects.
OERCommons
OERCommons is a clearinghouse of OER materials. Much of the content is user-created—options like lesson plans, handouts, and syllabi are available. Materials can be “stored” and shared by “groups” created on the site.
OpenStax
OpenStax offers complete textbooks. The site allows students to view the books for free online, download the pdf for free, and even order a “cheap” printed copy.
Merlot
Offers complete courses, textbooks, and individual lesson-type materials. Much of the material is linked to other sites. There is a comprehensive advanced search feature to help you dig down to specific material.
Wikibooks
On the main page, click on the “All Subjects” link near the top of the page to browse. There are many complete texts on this site. Most books offer a pdf version for download.
Open Textbook Library
Open Textbook Library provides many complete textbooks in a variety of subject areas. Each book has reviews and information about which colleges have adopted it. Most books are in pdf format.
MIT OpenCourseWare
MIT OpenCourseWare (OCW) is a web-based publication of virtually all MIT course content.