Assassin and Thieves' Guilds

In the West, there are a couple of small assassin guilds*, which may be part of the local thieves guild, based in major cities like Verella, Opal and Beryl.
Freelance assassins and duelists are found in many cities. Freelancers are usually connected to the city's criminal underworld and can be contacted through the right people ("I know a guy, who knows a guy"). Freelancers tend to be very good (but very expensive) to survive on their own.

Thieves' Guilds are common in many large cities, but are not as powerful and grandiose as they are in some fantasy novels and settings. Most guilds are "secular" in that they don't have close ties to any churches, though many thieves swear by Simyits (mainly in an attempt to avoid the attention of the god of Chaos). But since many of the actions of thieves are evil, it can lead to interactions with the followers of dark gods, especially Liart. It is rumored that the guild in Verella is massive and controls most of Tsaia, even overseeing highwayman and bandits in the countryside. And removing "unlicensed" competitors with their own enforcers or contracting assassins for the job.

In the eastern Arabic lands, the Moth Brotherhood of the Sorcerer Adventurers hire out as magical assassins. Rokugan has its ninja clans.

*-GM's note: The Nighthawks were stolen from Raymond Feist's Midkemia novels and wasn't an issue originally. But now I'm running games in a Midkemia setting borrowed from his books, and I put the Nighthawks in there where they belong. So I think I'm going to have to change the Aarenis assassin guilds to something else.