Khan Academy is a great one-stop shop for learning anything from math and science to history and economics. It offers a wide variety of subjects in the categories of Math, Science, Economics and Finance, Arts and Humanities, Computing, and Test Prep.
Khan Academy uses a unique teaching method, by combining YouTube lectures and examples with comments and questions located on the same page so users may further their learning experience with other active users. Along the subject track, users encounter skill check questions and interactive examples and demonstrations. If someone gets stuck on one of the questions, the page also offers the relevant video lecture so he/she may review that specific topic.