Eastern Market on Capitol Hill is fun, although I think only one or two of the vendors are actually farmers. My favorites are the cheese shop and the crepe cart. The crab cakes at the Market Lunch are also popular, though I've never tried them. If you're visiting in town, you can get here by taking the Orange or Blue line on the Metro to Eastern Market station. Exit the station and walk two blocks up 7th Street.

Takoma Park, Maryland, also has a nice farmer's market. They have more real producers there and also more organic produce. They are also one of the few farmer's markets I've found where you can use your credit card (especially appealing to miles collectors). To use your credit card, you go to a booth they've set up and purchase a set dollar amount. They then give you tokens you can use with any of the vendors at the farmer's market.