Vienna International Centre (VIC) houses the United Nations Office in Vienna and is the third largest headquarter of the United Nations (first being New York, second Geneva, and fourth Nairobi). It is conveniently located 10 minutes away from the city centre and is reachable by the U-bahn (underground).
VIC also accommodates the headquarters of the International Atomic Energy Agency (IAEA) and the United Nations Industrial Development Organization (UNIDO), plus other United Nations organizations like the Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO PrepCom), the Office for Drug Control and Crime Prevention (ODCCP), and other smaller United Nations entities.
My friend in Vienna plays bass and has a jam session in the UN building each week. Hence, I got a chance to see inside the building. It is quite fascinating. A huge building housing 4000 or so employees and a rabbit warren of corridors and offices. Worth seeing if you get the chance.
The United Nations and Secretary General Kofi Annan jointly won the centenary Nobel Peace Prize (2001) for working for human rights and to defuse global conflicts.
Some of the most important organizations like the International Atomic Energy Agency and The United Nations Industrial Development Organization, among other agencies of the UN are based at Vienna.