Skype!
I used Skype while traveling in Costa Rica this week, and the sound quality is amazing, like better than AT&T's cell service in lower Manhattan. I did an iPhone to iPhone, Skype to Skype conversation with a friend in California, and it was like he was standing next to me. Called Italy, it is just great.
For anyone who makes international calls or travels abroad, Skype is an absolute must. It replaced Notepad on my home screen.