One of the most interesting ideas I came across in the past few months was Metacademy. Its knowledge graph idea was less interesting to me than the idea that you could break knowledge up into prerequisites such that you could study without ever getting stuck.
Unfortunately, the website was abandoned before it ever really got to explore its real potential. It never added a fringe feature, which would see what you had marked as "learned" an tell you what you could study next.
I suspect its creators just got busy with other things. I hope they return to it one day.
Or, you know, I could figure out how to implement that feature and send a pull request. Anyone know a good introduction to Django?