Open Educational Resources (OER) are "teaching, learning, and research resources that reside in the public domain or have been released under an intellectual property license that permits their free use and re-purposing by others."
From The William and Flora Hewlett Foundation
More information on the specific licensing types and conditions creators apply to their work can can be found on the Creative Commons Licenses and Examples page.
According to the Bureau of Labor Statistics, Consumer Price Index (CPI) Database, the price of college textbooks has steadily increased over the last few years. This graph illustrates the rise in textbook cost from October 2008 to October 2018:
Supplemental, or ancillary, OER materials can include lecture notes, lesson plans, PowerPoint presentations, assignments, and activities.
Many universities have research guides related to finding OER and provide lists of various repositories and resources.
There are a multitude of OER out there to choose from, including textbooks, courses, multimedia, data, and supplementary materials. These can be found by searching regular search engines (like Google) by using certain keywords but it is much easier to find them through dedicated OER repositories or websites. The examples below are just a sampling of such repositories and websites.
David Wiley's 5 Rs of Openness,
see The Access Compromise and the 5th R for more information.