Eli's is the only place I know that is Kosher. But I do remember seeing a sticker with some sort of Kosher website on their door. It seemed as if you could search for kosher or kosher-friendly restaurants in the District. Perhaps a quick google search will help you out.
If you keep kosher outside the home, most of these places will work. One of my friends is pretty strict about keeping kosher outside the home and that works. If you are very hardcore, you don't really eat outside the house unless it's stated as a jewish/kosher restaurant.