A light sculpture graces one end of the Gallery Place/Chinatown metro station. I really like this sculpture—it brightens up the cavernous station very nicely. I know the DC Metro needs a lot more than better art (like more lines and trains and more money for maintenance), but just about every station could also benefit from a sculpture along these lines, as well.
For a look at how awesome Metro could but never will be, see Metro Dreaming from DCist.