The document provides a compilation of 20 essential open educational resources (OER) sites that every educator should be aware of, highlighting their significance in reducing textbook costs and enhancing personalized learning experiences. It defines OER and lists various platforms like Khan Academy, e-PG Pathshala, and NROER that offer free educational materials for teaching and learning. The resources included cater to a variety of subjects and educational levels, promoting accessibility and engagement in education.