Informally known as Ronchamp, the chapel of Notre Dame du Haut in Ronchamp (French: Chapelle Notre-Dame-du-Haut de Ronchamp), France completed in 1954 is considered one of the finest examples of architecture by the late French/Swiss architect Le Corbusier.
Le Corbusier at first refused the commission, not wanting, he said, to be associated with a 'dead institution'.
What a poorer place the world would have been!
More pictures here, the Wikipedia entry with yet more eye-candy is here.
For economists and everyone that hates them, David Galenson takes a look at the greatest architects of the 20th century. You can find the paper at the NBER website - for some reason I can't access it right now. Find out who was the - objectively measured, naturally - greatest architect of the 20th century, the greatest building, as well as what Le Corbusier and Frank Lloyd Wright have in common with Howard Roark, the most amusingly one-sided character in all of fiction.
More Galenson here and here.