Definition provided by OER Commons:
Open Educational Resources (OER) are teaching and learning materials that are freely available online to use, whether you are an instructor, student or self-learner. Examples of OER are: full courses, course modules, syllabi, lectures, homework assignments, quizzes, lab and classroom activities, pedagogical materials, games, simulations and resources contained in digital media collections from around the world.
Interested in taking a free course online from a reputable university to expand your knowledge? Here's a sampling of sites that provide a variety of online courses.