Ah, the joys of cleaning out old links. I first drifted over to this site while reading Moore's League of Extraordinary Gentlemen -- and promptly disappeared into it for two months. I warn you now.
Philp Jose Farmer, the great science fiction writer (although I believe he bristled at that categorization from what little I've read of him as an individual) came up with a unifying theory to the pulp universe. That is, WHY were these fictional worlds populated not sparsely by heroes and villains, but seemed chock-a-block with pulpy goodness? His genius was in creating an "Event Zero", the landing of a radioactive meteor in Wold Newton England in 1795. Passing royalty were irradiated, as were their servants, and form this origin group. From them spring Sherlock Holmes, the Shadow, Moriarty, Doc Savage, the Scarlet Pimpernel ...
All this would be enough, but the community that's seized on the idea is astounding. The challenge, using only works of literature or film qualified as "canonical", is to link as many genre heroes and villains into the Wold Newton universe as possible. The suggestions are, to a great degree peer-reviewed, and the inspired yet rigorous logic used to justify the connections is admirable. Crossovers are used, the timeline's extended not just forward but back ... frankly, I can't do it justice. Calling this fanfic is inadequate.
This is the Geek Ph.D.
Go get a crash course in your pulp and sci fi heritage, and find out how Peter Parker is related to Deanna Troi from Next Gen., X-Files to Captain America, and Kirk to James Bond.
The Wold Newton Universe.