Skype

http://ui.skype.com/home.html

Skype is free and simple software that will enable you to make free calls anywhere in the world in minutes. Skype, created by the people who brought you KaZaA, uses innovative P2P (peer-to-peer) technology to connect you with other Skype users. If you are tired of paying outrageous fees for telephony, Skype is for you!