I think it also depends on which region the people who are getting married are from.
For example it's very common for weddings in Valencia to include a bank account number on a separate card which is included with the invitation. Most people then simply make a bank transfer to that account annoting that it's for the wedding, etc. and the bride/groom don't have to worry about wedding gifts the day of the wedding.
I have been to two weddings in Valencia, and the first time I received an invitation, I was a bit taken aback, but quickly learned that it was quite common there. I don't know if Murcia, being along the Mediterranean coast as well, has a similar tradition. Such does not exist in Madrid.
Years ago, cash only was pretty much the norm. However nowadays, bridal registries (listas de boda as Jasmadrid mentioned) are becoming more and more common. It's always a good idea to ask your friend if s/he is registered anywhere. As Nevado suggested, just about anything goes...although it's highly doubtful that your friend will be expecting anything from you if you make the trip to the wedding.