The UN Secretariat building was designed by an international team of architects (most notably Le Corbusier and Oscar Niemeyer) and completed in 1950. It was the first important "International Style" modernist skyscraper in New York - exemplified here here by a simple, unadorned rectangle with reflective glass curtain walls on either side.
Glass box office buildings became almost cliche in mid-century NYC, but the UN remains unusual in being set apart in the skyline, uncrowded by neighbors.
I have mixed feelings about Le Corbusier's architecture (to say nothing of his urban planning philosophy - he clearly influenced Robert Moses), but I think the UN Secretariat building was one of his successes.
An aside: If you look at the full resolution version (downloadable on flickr), you can see the HF amateur radio antenna on the roof. Nerds are everywhere, even/especially at the UN. There's also a family taking a group picture on the street in front.