The Phillips Collection is a fantastic private gallery near Dupont Circle with a great collection of modern and post-impressionist art. I visited on Sunday with my girlfriend and was impressed with their exhibits. One of my favorites was an entire room dedicated to painter Mark Rothko.
Although I've never been a big modern art guy, Rothko's work appeals to me. (Before I go any further, let me add the disclaimer that I know nothing about art. Nothing.) Most of his paintings use only a few colors distributed in horizontal bands, and he's most well known for his use of subtle color variations. The most impressive thing about Rothko's paintings to me, though, is their scale -- most of his canvases are six, seven, or eight feet tall and five feet wide.
While Rothko rarely commented on his paintings' meanings, he always insisted that museums display his works without frames (he always painted the edges of his canvases) and in such a way that viewers could stand very close to them. The room dedicated to his work at the Phillips Collection must fit Rothko's specifications perfectly -- you walk in, stand in front of the paintings, and they overwhelm you. Definitely worth the walk down.