There has been an uptick in notices about Fanfic (Fan Fiction) of late, particularly involving an aspiring author who tried to offer her Star Wars fan fiction for sale on Amazon.com (The lawyers stepped in and hilarity commenced). I've always thought of fanfic as a child of a lesser god, at best warm-up excercises for real writing. But Theresa Nielsen Hayden really nails the nature of fan fiction on the head here.
I think the operative line is this: "Fanfic is a legal category created by the modern system of trademarks and copyrights". And she's right. The borders of the fan fiction enclosure are determined by who owns the property and whether it is still enforced. It used to be the Fanfic was easily identified by mimeo paper and typoes. Now with the Internet and print-on-demand, those easy tell-tales are missing, and the phrase, while it retains a legal meaning, has no literary meaning.
Depending on where you set down those borders, I have either written no fan fiction in my life, or written almost exclusively fan fiction. The work on Starcraft and Warcraft books were licenses, but while I expanded those worlds, I did not generate the original properties. The FR, DL, and Magic: The Gathering books I've written have been in worlds/Intellectual Properties I've helped develop, but there are many original hands involved in the mix. And most recently, I've written a short story for The Further Adventures of Beowulf (coming this fall), which could be considered fanfic except for the fact it has a major publisher and the Geats don't have a legal staff anymore.
Once sensitized, I started going through my bookshelf for other examples: Shadows Over Baker Street, which merged the Cthulhu and Holmes mythoi, is only done well because the writers engage with both universes. Pride and Prescience, a charming book by Carrie Bebris, takes Jane Austin's Elizabeth and Lord Darcy and turns them into detectives. The Mad Merlin books by J. Robert King build heavily on not only the Authurian mythos, but other concurrent legends and works as well. All build directly on predecessors, using characters created by others.
Fan fiction, in the broadest sense, all. Because you have to be a fan in order to put up with all the pain involved with writing.
More later,