The Smithsonians in DC are free. They include The National Gallery of Art which is always a crowd pleaser and the Hirshorn, a modern art gallery that is weighted more towards mid rather than late twentieth century.
Also includes the Freer and Sackler Galleries of Asian arts.
I absolutely love the Smithsonian museums. They are wonderful. I have also walked by the Corcoran so many times but have never gone in - I'm planning on going one weekend soon, but it's a DC art museum that's also supposed to be great.
