Several Examples:
Sembia: Mirabeta Selkirk of Ordulin is a highly influential noble, who has her hands in everything from slave trafficking to illegal goods. She and her five sons are quite nasty individuals, who just happen to be related to Kendrick Selkirk, Overmaster of Sembia.
Westgate has the Night Masks, the most influential thieves' guild and guild of assassins within the last 400 years of Faerun's history, since Westgate controls so much of east-west trade. They run a good portion of the government, and the city.
Calimshan has a city with close to one million souls, most of which live in poverty and/or squalor. The Shadow Thieves run almost three quarters of the city with an iron fist, but hidden within a velvet sash. However, they do not have as much wealth or prestige as the merchants of Sembia or the 'Masks of Westgate.
If running an organized crime campaign, your players would have to be prepared to either take part in or at the very least observe some very brutal and atrocious events - otherwise, it would be a very watered-down type of organized crime, with the violent parts hidden from view. This type of campaign would also be much more political in nature than a typical D&D campaign, because Organized Crime is about power - who lives, who dies, and who profits from the business that does go on. Not all O.C. is about violence and death, but the element should always be there, in the background, reminding the PC's of the wages of failure.
