Khan Academy is one of the early online learning sites, offering free learning resources for all ages on many subjects, and free tools for teachers and parents to monitor progress and coach students. You can learn many subjects here like Physics, Chemistry, Biology, Health & medicine, Electrical engineering, Art history, Grammar, Music, US history, World history, Computer programming, Computer science, Hour of Code, Computer animation, Microeconomics, Macroeconomics, Finance & capital markets, Entrepreneurship, Early math, Arithmetic, Algebra, Geometry, Trigonometry, Statistics & probability, Calculus, Differential equations, Linear algebra, Math for fun and glory etc. You can learn absolutely free of cost. You just have to sign up/register and choose whichever subject you want to learn. They have video tutorials too to make things easy for you.