SR Crown Hall, Illinois Institute of Technology, 1950-56. Ludwig Mies van der Rohe. The architecture building as IIT, designed for flexibility. The partition walls can be moved, which allows the space to be converted from studios to galleries to performance spaces. The thin roof is an amazing engineering feat.
Quote taken from Mies van der Rohe's speech 'Architecture and Technology', given on April 17, 1950 in celebration of the addition of the Institute of Design to the Illinois Institute of Technology. The whole speech can be found at the website of the Mies van der Rohe Society (http://www.miessociety.org/speeches/).