Here's a neatly categorized and tantalizing list of medieval urban professions, including the criminal trades.
silk-snatcher – one who steals bonnets
stewsman – probably a brothel keeper – "since the words stew and stewholder both mean a bawd, I'm guessing that a stewsman would be a brothel-keeper as well. Whether bawdry counts as a criminal activity varies at different times and places."
thimblerigger – a professional sharper who runs a thimblerig (a game in which a pea is ostensibly hidden under a thimble and players guess which thimble it is under)
eggler – an egg-merchant
fool
Knifeman – one skilled with a knife; specifically, a soldier trained to disembowel horses
What did people do: in a Medieval City?
(via Kottke)
