The one which impressed my Spanish friends the most was from the Guardia Civil post in Tamarite de Litera near Balaguer. The Guardia Civil, seeing me walking in an unshaded landscape at 36°C, stopped me and drove me into the pueblo (I have always had a tendency to coöperate with men with weapons) where they made me orange juice and stamped my credencial.
Generally, all churches have sellos, and so do the ayuntamientos-- in remoter places, the ayuntamiento seal will be the easiest available. I cannot recall if it is compulsory or just strongly advised to get two per day in the last 100 km, but as Laurie/Peregrina2000 points out, it's fun to look at them months later. As well, one of my overly-legally-minded friends points out, they are proof of your alibi that you were in Spain, should you have need of one.