Dupont Circle is one of my favorite places to skate.  Going down 19th St from my dorm room at GW, there's lot of fun spots to hit before actually getting to the circle.

Actually at the circle, there's some weird grate things that have nice rails that can be ground, and there's also one of my favorite spots in DC to skate, the brick banks.

As you can see, little metal bars have been put up to prevent the super long wallride that you could have, but there banks still function perfectly for doing all kinds of fun tricks.  Hopefully in the future, the transition will be smoothed out with a little cement so you don't have to ollie to get up them.  The only draw back is that to get any speed, you need to start across the street, and on a Friday or Saturday night, Dupont Circle has a crazy number of people and cars all about, so you have to watch yourself.  It can be entertaining to come here on the weekend so you have some entertainment while you're sitting around.

Update: This spot is no more. Over the summer they came in and put up this metal bar, effectively making it pretty hard to skate here. Another unfortunate case of people trying to stop skaters from skating their terrain. What bothers me is that this wasn't even some curb that was getting marked up, when the banks were still brick, there was no sign oncesoever that people skated there. But anyways, you can still do some fun stalls while holding on to the bar, just be a little creative.