Last time I went I got a Yoigo card, because they offer 3G data plans on their card (fixed max spend per day, regardless of how much data you actually use).

But the previous time I got a Happy Movil card, which does not offer any data service, just voice. However, calls to the US were only 6 cents per minute -- calls within Spain were more expensive, around 19 cents per minute. There was also some deal where if you load 10E on the card, you got 5E of bonus money on it. For just voice, especially calling the US, that card is hard to beat...