Athena
Athena aimed to accelerate human learning by mapping knowledge as a directed graph of concepts. Learning a concept depends on understanding prerequisite concepts - a map would let you point at what you knew and where you wanted to be and see the paths to get there. Three of us, Juan Batiz-Benet, Ali Yahya, and myself, built an online wiki graph editor to construct this. Each node (a concept) contained educational materials sourced from the interwebz.
We built a general-purpose web media remixer so the relevant piece of an explanation could be used without including the whole thing, or so that complementary components could be joined in a larger explanation. The remixer could clip, loop, and splice videos from YouTube and Vimeo, stitch together images, and incorporate raw text and PDFs. (It had it's own name, Acorn.)
We built graphs for computer science and math classes at Stanford and had students using them.
Among our mistakes was a lack of focus on developing users. We saw a world in which independent learners online could take custom paths through knowledgespace, finding wherever they went connected trails of content set free from the one-size-fits-all rigidity of a lecture series, but we failed to bring it about.
If you are working on this dream, please let us know. We would love to help.
Some of our code is opensourced - the remixer core, some libraries - but most of it isn't. There are some private things in the rest, but if you're interested let me know and I can give you access.