Las Tunas in Clarendon
I've been there about three times, but not in the last year and a half or so. I've always liked it, the food is solid Salvadorean and Mexican, not showy. I'm not really recalling anything as being a real standout, but I'd put it on my list of authentic, decent places - not too expensive either. Mostly Spanish speaking and local so sometimes you get looks when you come in the door. The bonus is you can visit the Indian Spices and Appliance shop across the street after your meal - that place is just bizarre.
Good El Salvadorean/Mexican dive (great margaritas) in the district is Casa Juanita's at 11th and I.