"Brushstroke," by Roy Lichtenstein, in front of the Hirshorn Museum and Sculpture Garden on the Mall in D.C. According to this article about the sculpture's dedication:

The Lichtenstein work, “Brushstroke,” is a 32-foot-high-by-20-foot-wide sculpture of a dynamic brushstroke of paint, and it has been installed in the museum’s main plaza.

I love Lichtenstein, so I was happily surprised to find this piece as a permanent installation. About a year ago, the Corcoran Museum of Art featured another huge Lichtenstein sculpture (another brushstroke, I think) outside on the corner of its block, but it's since been replaced by other exhibits. The National Gallery also has several paintings that are usually on display on the lower level of the East Building.