Ooh! I have two HP/Discworld recommendations:
Minimal Grounds For Disaster by After the Rain. Remus Lupin has problems. He's a wizard. He's a werewolf. He's somehow got himself very, very lost. And he's just been arrested by the Ankh-Morpork City Watch. Things can only get worse...
Harry Potter and the Improbable Use of Chaos Theory by debchan. Take one part Discworld, one part Harry Potter, mix well and...
ETA: Fireworks by sam_storyteller. Remus Lupin pays a visit to his alma mater -- the Unseen University, Ankh-Morpork -- and runs across one Sergeant Angua, off-duty and pissed off.
I'm looking forward to the Discworld