The term open educational resources (OER) was first introduced at a conference hosted by UNESCO in 2000 and was endorsed in the context of providing free access to educational resources on a global scale.
OER include a varied range of materials: books, case studies, courses, journals, primary sources, assessments, assignments, reference materials, tutorials, tests, and more.
The 5 R's - Source http://opencontent.org/definition/
... Access, Equity and Pedagogy.
Use OER instead of commercial textbooks if it’s pedagogically fitting.
OER release should be subject to ongoing Quality Assurance process which is transparent and fair, however this is not always the case.
Without preservation of these often one-time start-up OER projects, the initial funding will end after a few years and sustaining the resources will be expensive, the resources may become outdated and the quality gone.
Many do not understand the potential of OER and feel that it threatens their ownership of intellectual property. Also, it takes time to creat and/or locate existing OER.
Use the Advanced Search option in Google to look for OER.
Scroll down to 'Usage Rights' and select Free to use, share or modify, even commercially.