Free calls on Vodafone Spain (Summer specials)
 

For visitors to Spain this summer, Vodafone is doing a deal where for a one off payment of €6 you can make free calls to other Vodafone Spain users. Calls are limited to 150mins per call and this promotion is valid all summer long (until 21st September). Perfect if you are considering buying 2 or more prepaid SIM cards just for staying in touch with others in your travelling group. http://www.vodafonesites.com/suenamejor/

(PS. As my Spanish is not perfect, I may have missed some conditions).

I believe Movistar has a similar offer on, though I haven't found it yet.

Just found the Movistar deal: http://www.movistar.es/verano/
This also works on their prepaid cards. It costs €5 and unlike the Vodafone deal you can only talk in the off peak times.