Kahn Academy provides free detailed YouTube videos covering various topics in math, science, English and history for all grades from elementary to college. Kahn Academy also provides free SAT and AP test prep.
Wolfram Alpha covers a broad range of mathematical subjects including Algebra, Calculus, Statistics, and much more. Rather than simply providing students with an answer, Wolfram Alpha will show the intermediary mathematical steps that were used to get to the correct answer.
This provides reviews of classical novels. In addition, provides information regarding themes, characters, metaphors, and symbolism throughout the book and summaries of important topics from a variety of high-school class subjects.