Neil Gaiman's short story Baywolf is a retelling of Beowulf in Baywatch decorations. In a cyberpunk setting. And the main character is a werewolf who works as private detective. And the villain is implied to be the Creature from the Black Lagoon. He got the idea when more people than is healthy thought he was doing a baywatch episode every time he spoke about the project.
"A Study in Emerald" has Sherlock Holmes meet the Cthulhu Mythos. The real brain-bender is that the Great Detective in the story is Professor Moriarty, and Holmes and Watson committed the murders he's investigating. Weirdest of all, it was completely plausible in context.
The story is available free of charge on his website, complete with the original artwork here (PDF)
Gaiman's "Snow, Glass, Apples". It's a retelling of Snow White in which the titular character is a vampire. And in the end, we learn that our narrator is narrating this from an oven—apparently she's a course in Snow White's wedding feast.
And since we're talking about Neil Gaiman, there's "The Problem of Susan", Gaiman's response to C.S Lewis' Narnia, in which Aslan and the White Witch (in a dream sequence) end up joining forces, turn on the children, and kill them all. And Aslan eats them. And then Aslan and the White Witch have sex. And the decapitated head of one of the children is forced to watch them do so. Yes, you did read that correctly.