Harry Potter and the Improbable Use of Chaos Theory Crossover between Harry Potter and Discworld. Granny Weatherwax and Nanny Ogg end up in Hogwarts and start sorting a few things out. Absolutely hilarious - if you're not laughing out loud at least twice, you're already dead.
Hellblazer : Hogwarts John Constantine becomes the new Defense Against the Dark Arts teacher. WIP, but strongly recommended.
This has been a public service fic recommendation.