A Cold Day in Hell
Undoubtedly this is the largest organization in the world. The guild strives to keep crime under a certain level; the level depending on the size of the town and its relative wealth. The guild also regulates a secret messaging system that can only express very simple concepts. It is expected that any thief, beggar, prostitute, and the ilk has to report to the guild; only if they plan to work there for longer than a week. The guild typically gets a cut, but in exchange there are no turf wars and much less violence between thieves. They also require a fee to join in each area, which usually means just that city. Also, there are standard expenses that aren’t waved; fence cuts, information prices and (as you would expect) the prostitutes. However, you can usually get decent prices on shabby places to stay, shabby ale, and shabby food.
The guild takes care of its own. While allowing professional hitters is up to the individual guilds itself, they certainly have the resources to get one at any time. Of course, there are fates worse than death; especially when facing people who do not exist and call no place home.
Personal note: I really like the concept of the thieves’ guild, so I use it in almost all my games. It only makes sense that crime would be regulated. After all, if they just ran rampant, then guards would double and rogues would be almost extinct.