In all the developed-to-developing countries route. So all your Top 25–30 countries that receive remittances, also have very well established hawala/hundi routes.
This is a natural by-product. It exists because invariably, there will be a grey channel demand.
But they also exist for a reason most people tend to forget. Capital outflow restrictions in the receiving countries. With the air of hawala networks, people within a developing country can easily contact ground agents in their country to send payments to people or businesses in the US (for example).