In a city like DC where many people are transient, GLUT has the right idea. It's structured as any traditional co-op. There's a huge bulk selection - dry and liquid, as well as a great selection of dried fresh fruits. They've got local milk and cheese, which is hard to find in DC. Basically it's all the amenities a conscious eater could ever want, but they recognize that you may bot be here forever and don't charge membership fees. The prices aren't just reasonable for a specialty market, they're also just good for DC in general. Not much parking so ride your bike or take the G8 up Rhode Island.
Cons: Out of the city