Context (you don't really need it, since the comments begin with the quote that sparked it off)
"If people like Lee and Claire and myself are wrong - then why is there not ONE full-time writer who also engages in fanfic?" - Paul Guyot.
Hm, let me introduce:
W. Shakespeare. - Never thought up an original story on his own and his OCs were limited to supporting players. Also very prolific RPFer.
J. Milton - The original Bible ficcer.
Dante A. - RPFer who went to Hell and back for his art.
G.B. Shaw - Greek myths ficcer. His work has been labelled AU.
C.S. Lewis - decided to tell Apuleius' Cupid and Psyche from the point of view of the evil sister.
C. Marlowe and J.W. Goethe - competing writers in the Doctor Faustus fandom. Marlowe's version is a bit lame, but Goethe's sequel totally sucked.
Brothers Grimm - worse plagiarisers than Cassie Claire. But even more famous than her.
Mercedes Lackey - The Black Swan. A retelling of Swan Lake from the POV of Odile, Rothbart's daughter.
Tom Stoppard - Rosencrantz & Guildenstern Are Dead. Hamlet fanfic.
Robin McKinley - Beauty : A Retelling of the Story of Beauty and the Beast
Jon Scieszka and Lane Smith - The True Story of the 3 Little Pigs A retelling of 'The Three Little Pigs' from the Wolf's POV.
Plato: His Dialogues were so RPF.
John Gardner: Wrote the first part of Beowulf from Grendel's point of view. (Totally kickass existentialist novel, by the way.)
Ray Bradbury: Also wrote RPFs involving a great number of dead authors.
August Derleth: Lovecraft's own fanboy that kept the mythos going after Lovecraft's death. He also fanfic'd a Sherlock Holmes novel.
Stephen King: Speaking of a guy who penned a Sherlock Holmes fic--he wrote one from Watson's POV. And also dabbled in Lovecraftian mythos.
But those latter three probably don't count since they're GASP, not working in 'real' writing genres.
Oh man, we'd be here for hours if we talked about all the Lovecraft fanfic writers.
Neil Gaiman, Robert E. Howard, a bunch of others too numerous to mention.
Speaking of Stephen King and RPF, he wrote a short story about a bunch of dead rockers.
Angela Carter - whole book full of Brothers Grimm fanfic. Fanfic of fanfic! OMG!!
Welcome to the great literary tradition!