If you live in the US and you make a lot of internation phone calls you may want to check out http://futurephone.com. This is a free service that allows you to make free international phone calls to over 50 countries by calling their gateway access number. The international portion of the call is free, you pay only the call to their Iowa based access number, which with the increasing popularity of unlimited long distance mobile or landline phone plans, makes the service entirely free.
I have not yet used the service to test the quality of the service, but it is definately worth checking out. I hope that they will expand the service to offer gateway access numbers in other areas such as here in Italy!! If you have used the service let me know what you think.