A couple dozen boulders in Northwest Branch Park in Silver Spring. Unlike most bouldering close to DC, many of these are actual boulders with top-outs.
The difficulty level ranges from 5.1 to V11 (you don't have to be a "real boulderer"). The area's best features, though, are a two-minute detour off the Washington beltway, a parking lot that doesn't fill up, and a one-minute approach to most of it along a maintained path.
From the Washington Beltway east of Route 270, take the exit for Route 29 North one and a half miles and park on the right (east) side, in the park parking lot (next to a brick building) just south of the Trader Joe's. The path east to the boulders starts in a corner of the parking lot.