[jason taylor]

Joint Stock Corporations: These were actually invented in the late Middle Ages and contained most of the refinements we know today. Usually they would however be a catspaw for a powerful family or group of such rather then a self-existent formation.

Alberghi: Somewhere between a clan and a guild. Or rather they were what might be called artificial clans. They were family confederations that controlled a section of urban area as well as some commercial assets. They would likely have a private citadel as well as some public facilities such as a wineshop or bathhouse held in common.

Speaking of that, Chinese clans are also a model. These tend to be scattered but have a headquarters, usually a shrine, where geneological records are kept. I forget what the other details like how combined polices are decided are.

Secret Societies: These can vary from the mundane to the sinister. In some areas they might form an alternate government. In other areas enough officials are members that it is another face of government. The thieves and assassins guilds already give hints of this.