Neil Gaiman is an English author living in America who is famed for his dark, twisty short stories, his Sandman graphic novel series, and several full-length novels, primarily in the Urban Fantasy genre. These include Stardust and Coraline, both of which have been made into movies. He also co-wrote Good Omens with Terry Pratchett. Good Omens has spawned an OFU, the Good Omens Fanfiction Summer School.
Neil Gaiman as an author is particularly interesting for fanfiction writers in general, in that many of his stories are retellings and reinterpretations of old tales, sometimes turning them completely inside out and making something new and awesome rather than just leaving broken pieces of the old. He has made new stories out of Snow White, Beowulf, The Chronicles of Narnia, and a crossover between Sherlock Holmes and the Cthulhu Mythos.
He has a website here.