The Smithsonian Institution
http://www.si.edu/newstart.htm
The 150-year-old Smithsonian Institute comprises the National Portrait Gallery, the National Museum of American Art, the National Air and Space Museum, the Sackler Gallery, the Cooper-Hewitt Museum of Design, the National Museum of American History, the National Museum of Natural History and more. This site defies categorization, as it has a lot of everything.