There a variety of services that offer free international calls, with different requirements for each. Some notable examples are Skype, EvaPhone, Rebtel, phone2phone and Talk-O-Naut.
Of the services listed, EvaPhone, phone2phone, Skype and Rebtel are all available for use on a computer. Evaphone allows users to make free calls to dozens of countries but has a maximum time limit involved. If users exceed this limit, there is a charge. Phone2phone is a similar service, but it has a maximum time much greater than that of EvaPhone. Both of these services require users to watch an advertisement before making the call.
Skype has no set time limit at all, but each user must have a Skype account in order for the call to be free. If a user calls a cell phone or landline from a Skpe account, there is a charge.
Talk-O-Naut is entirely based on mobile phone use. This service connects with Google Voice in order to call any other phone for free. Skype also offers a mobile phone app to use for international or domestic calls. Although it is free, users must still call another Skype user and are charged for the data used if the phone is not connected to Wi-Fi.
Unlike the others, Rebtel is a calling service that allows unlimited free calls to certain international locations. However, this service comes with a monthly charge, similar to most cellular carriers.