I'd like to start out with a few reccomendations:
Apples: A Discworld/LotR crossover, featuring what happened to Shadowfax.
The Detective and the Diplomat: Discworld/Sherlock Holmes: Vetinari and Holmes have a few mysteries to solve...
Disc Wars -Episode 4: SW/Disc: There are two more in this series, in which the Disc characters parody the classic Star Wars trillogy.
Watchmen in Black: MIB/Disc: MIB come to Discworld to hunt aliens. Sam is not amused.
Evidence: Silm/Disc: Samuel and Death have a little surprise for the sons of Feanor.
Harry Potter meets Ponder Stibbons HP/Disc: "The Boy who Lived comes face to face with the man who never went out again."