During my trip to Washington DC I went to the American Art Museum and the National Portrait Gallery which are both in the same building. The building features an amazing central courtyard that was finished in 2006. It is the third oldest Federal building in the city. The American Art Museum features wonderful modern pieces as show below.
The National Portrait Gallery has a wonderful collection of Andy Warhol pieces as well as classic portraits of the founding fathers such as Benjamin Franklin. Entry is free to both galleries.