Open Educational Resources (OER)
OER are teaching and learning materials (text, media, digital, etc.) that you may freely use and reuse, without charge. OER often have a Creative Commons or GNU license that state specifically how the material may be used, reused, adapted, and shared.
American Institute of Mathematics Open Textbook Initiative
BC Campus Open Education Resources
COSC 1301 Introduction to Computing
George Mason University Metafinder