Vodafone Logo (Photo credit: Wikipedia)
Yoigo (Photo credit: Wikipedia)
I've been coming to Spain for the last half dozen or so years, and have long been a fan of Yoigo, the Telia - Sonera backed mobile operator, who is fourth behind Movistar, Orange and Vodafone, and feels heat from upstart MVNO MasMovil. Given the competitive market here, I've tried them all and find Vodafone's 29 Euro for 1 GB over three months to be the best plan available.
Yoigo though is best for your smartphone, with Movistar having the best coverage if you leave the major cities.
Mas Movil takes USA credit cards, but you need to go through some hoops to set up that, but once you do, you're golden. As an MVNO they use both Voda and Orange networks at last peek, and their service is rock solid.
All of this said, using services like Skype and VoIP are not usually allowed on Yoigo, and they have a daily cap on your consumption.