The United Nations (UN) is an intergovernmental organization to promote international co-operation. As of this writing, there are 193 member states. The headquarters of the United Nations is in Manhattan, New York City. Further main offices are situated in Geneva, Nairobi, and Vienna. The United Nations organization has it’s own postal agency: The United Nations Postal Administration.
I’ve visited the UN building in the New York City, and sent myself a couple of postcards.
The UN building is the blocky building on the left:
And they have their own stamps, of course. The one I got featuring Borobudur Temple in Indonesia, which is UNESCO World Heritage Site: