The Khan Academy is an non-profit organization who’s mission is to changing education for the better by providing a free world-class education to anyone anywhere. There are over 2100 videos with subjects ranging from first grade Arithmetic to Differential Equations and basics of motion to Mohorovicic Seismic Discontinuity. Their library includes math, science, economics and history. There is SAT prep, GMAT prep, CAHSEE prep and IIT JEE prep. If you got through all 2100+ subjects you would be really smart.