Unsurprisingly Harry Potter tops the list and no doubt JK Rowling finds this flattering...because it is.
Anne Rice OTOH is not fond of fan fiction. She has apparently copyrighted her characters and pursued the writers of fan fiction vigorously - the story that she drank their blood is just a rumour AFAIK
Amusingly the writing team of Buffy the Vampire Slayer whilst welcoming the fan fiction were forbidden to read it in case they inadvertently used ideas from the fan fiction in the series proper.
As you can tell from this I think it is a good thing...the quality is very uneven but using existing stories as a starting point is a time-honoured method. Was Shakespeare writing "fan fiction" for example ... well a less polite person might actually have called it plagiarism....
Do you have any opinions about fan fiction?