Tuenti is excellent

[wco81]

Here it is:

http://prepaid-data-sim-card.wikia.com/wiki/Spain

And here's the account about Tuenti and foreigners:

Extra rules for foreignersEdit
Legally they are not allowed to discriminate foreigners, but tuenti has invented some special rules for them: In physical stores like Movistar shops you can do the registration only by showing your passport. Unlike holders of Spanish ID cards (DNI, NIE) you'll have to pay 25 € and the only option to choose is the 7 € package. The rest will be given as a 18 € card balance.

This policy is not enforced if you order it online. You can use your passport ID or Spanish ID cards (called DNI and NIE) online and pay with a credit card or cash-on-delivery and choose any package you like. They'll send it to any Spanish address like your hotel.