We used the Carrefour Data Solo Sim for Spain.
You need to ask for a "Tarjeta Carrefour Internet Móvil Prepago" (= prepaid data-only SIM):
SIM costs 5 € without credit. It's a data SIM for data and SMS only, no voice calls.

Tarifa diaria: default plan, no activation is necessary,
1.21 Euro per day (= 24 hrs), 100 MB at full 3G speed, thereafter speed is reduced to 128 kbps. You are charged only for the days that you connect. A 3 € per month minimum spending applies (as long as you have credit).

Bono 1 GB add-on: 10 € for 30 days, 1 GB at full 3G speed, thereafter speed is reduced to 128 kbps. Activation: by sending SMS to 22864 with "ALTA BONO 1GB". Deactivation: by sending "BAJA BONO 1GB" to 22864. Data balance check: by sending "BONO 1GB" to 22864 (5 free SMS per month)

Bono 3 GB add-on: 20 € for 30 days, 3 GB at full 3G speed, thereafter speed is reduced to 128 kbps. Activation: by sending SMS to 22864 with "ALTA BONO 3GB". Deactivation and data balance check: see above.
In Carrefour megastores it can be bought using a foreign passport and local Spanish (hotel or campsite) address for registration only. Note, that there are two different SIM cards on sale: the INTERNET móvil with data-only and the regular prepaid card Tarjeta prepago Carrefour (= prepaid voice and data SIM)and a different tariff.
Credit can be displayed by calling *113# (for free up to 5 times per month). You can add credit online or in Carrefour supermarkets. Some tabacco shops and the Carrefour chains Día and Clarel offer credit for Carrefour Móvil too.
This card runs on the Orange network.