There's no shortage of suggestions, but we just returned and I highly recommend going to Astelena Taberna, where you should have the tapas from the bar as well as those made by the woman in the back, which included a foie gras tapa. The bill for two of us was 40 euros and well worth it.
Even more creative are the tapas at Alona Berri, where the chef won the "best tapa of 2008" award. We went there on a Sunday afternoon and loved everything.
Both places are extraordinary and memorable, fairly priced and friendly.