I recently picked up Vols 1-5 of "The Man-Kzin Wars" at a swap meet for about a buck apiece. I've read Vol 1 and am eagerly diving into Vol 2. What's really fun about this is that here you have a bunch of pro-fic writers delving unapologetically into someone else's universe...and the creator of that universe is not only okay with that, but enthusiastic.
Larry Niven, who is a creative genius, isn't threatened by others writing Known Universe stories. Lee Goldberg, who *cough* isn't much of a creative genius, is. Granted, the people playing in Larry's sandbox are well-respected authors in their own right and not a bunch of amateurs like me...but somehow it gives me warm fuzzies to realize that, as long as I don't butcher the KU, Larry probably wouldn't have much of a problem if I decided to write fic in it.