Related to @mden's Idea: Tree of knowledge.
Chrono: chronological inventions and academic breakthroughs of mankind as a dependency graph. This is a lingering idea that has been coming back to me a couple times a year over the last decade or so.
What if there's a kind of semantic wikipedia that is built upon a dependency graph of inventions and academic breakthroughs. What led to the invention of the internet, to nano-tubes, etc? How cool would it be from an education standpoint to be able to jump back in time and see invention upon invention replayed (with backgrounds on how these breakthroughs came to be) up to today.
Check out what led to invention X (the galaxy S you're reading this on), played back . Or reversely, lookup which inventions were build (transitively) upon the discovery of Y. You'd also finally be able to answer definitively who was more important: Tesla or Edison ;)
Socio-economic backgrounds, anecdotes, etc. what led to invention X, and how X was important for Y, etc. An interactive "Short history of nearly everything"