In addition to the official emissions, there is also production of the semi-official, and private postage stamps. Semi-official releases usually appear with the permission of the State Post Office, or at least, in the absence of objections from it's side.
Semi-official stamps appeared in the period when the delivery of mail by air was still in the experimental stage. The reason for their release were mainly the test flights, aviation week and other events. For example, in 1912 - 1913 the Bavarian Aero Club, receiving the official permission on the organization of airmail, issued it's own stamps. The correspondence was supposed to be given to the offices of the Bavarian mail in the usual manner, but along with the usual stamps it had to be marked with the stamp of the club (it's value was 25 pfennig).
In most cases, the private stamps were produced at the places, that were outside of the areas, served by the public mail. They were used to pay for the delivery of the correspondence to the nearest governmental post office. There is, for example, a stamp of Tierra del Fuego, released in 1891 by the gold-miner Hulie Popper. The correspondence, marked with this stamp, was delivered from the mines to the post office of Punta Arenas (Chile).
On most letters of the overseas countries - until they became a members of the Universal Postal Union, or until their own mail started to deliver the international correspondence - there can be found marks of the postal payment, produced by the private firms. There are well-known, for example, the private "Brice stamps", which for many years were in circulation on the island of Trinidad. They were released in 1847 by Captain David Brice, the owner of the ship "Lady McLeod", plying on the route "Port-of-Spain - San Fernando". Face value on the stamps was missing, even though they were sold for 5 cents per piece. One of the most popular postal services during the "gold rush" in the United States was the American company "Pony Express", carrying out the express delivery of correspondence from the territories of the gold mines of California to the other states of America.
Generally in the world there are so many private stamps, that it is not likely, that they all can be enumerated. It got to the point that the Alpine hotels in Switzerland were issuing their own stamps. They were used in the winter time to pay the services of carriers, that were delivering the correspondence of numerous hotel guests to the nearest post office. Prior to April 1, 1900, in many German cities, along with the state post offices, there were a private ones, whose activities, however, were limited to the delivery of a local mail. They appeared because the Postal Act of July 5, 1852 called for the state monopoly on a mail delivery of messages only between settlements. This fact and the lack of telephone service gave rise to a large number of local mail services, that were delivering the correspondence within the city by the private postal companies.