Small restaurant for tapas and "raciones" (larger tapas). It's usually full. Try to get there early, write your name on the chalk board and wait to be called with a caña (beer) or shandy ("clara"). Definitely worth the wait.
Recommended: the Quail (codorniz) fried eggs with "Jamón serrano", the "calamares (squid) del campo" and "solomillo al whisky" (sirloin steak). Oh! And it's cheap prices for amazing quality! =)