[nsajko]

Math Academy seems like it could be a game-changer for learning, it's a paid service and in beta-testing. They only cover math, though, at least right now.

In addition to presenting the topics and providing exercises, MA automates some tasks that are usually left to the learner, like spaced-repetition and tracking down prerequisite topics. They have a detailed DAG of math topics connected with dependency relations that enables this. Basically automation of a lot of what a tutor would provide.