Khan Academy produces expert video lectures and interactive learning modules on subjects ranging from Mathematics and Computer Science to Economics and Art History. A non-profit organization funded in part by The Bill and Melinda Gates Foundation and Google, Khan Academy is totally free, and always will be. Some school districts around the country have already begun supplementing their normal curriculum with Khan Academy content and exercises. Khan Academy’s math coverage is particularly strong, covering the subject from the very basics of Arithmetic all the way to college-level courses like Calculus, Linear Algebra, and Differential Equations.