As you probably know, the United Nations Organization’s Headquarters are located in New York. It’s not the coolest place to visit, as it is not the tallest nor the most beautiful building in Manhattan, but still an important place to see for what represents (another topic would be how effective it really is, but this it out of the scope of this post).
These headquarters are located in Manhattan, next to the east coast of the island, and it is international territory. There is no proper border (and you don’t get any stamp on your passport when you get in), but only international law rules here. They also have their own security, firemen, post office, etc.
If you happen to go to New York: don’t take the guided tour here: just stay outside, or in the main hall. We did so, and are quite disappointed. The only interesting part in my opinion was the General Assembly, but it lacks of some essentials: I would have enjoyed some history about the UN and some important decisions taken there, and also some other important sights, like the Security Council (this is a hot place nowadays).