The CK-12 Foundation is a California-based non-profit organization whose stated mission is to reduce the cost of, and increase access to, K-12 education in the United States and abroad. CK-12 provides free and customizable K-12 open educational resources aligned to state curriculum standards. The resource they have assembled online is phenomenal.

Their service comes up anytime one googles “online textbook”, and as such, teachers are likely to find some excellent content to overhaul or extend exsiting assignments and textbooks already being used.