In the Washington area,
California Tortilla has great Mexican food (with a Californian twist) . Cheap, fresh, healthy and fast.
The quality does seem to vary by location a bit. The Bethesda and College Park locations are excellent, while the Rockville one is pretty inconsistent. They also have a few airport branches now as well - at BWI and IAD.
Also,
Lebanese Taverna is slightly more expensive (maybe around $7-10) but the food is excellent, especially the chicken shwarma.