Skype is very cool. You can make call to anywhere in the world for free, as long as the download the software.
Skype is a "softphone" - a software-based telephone that uses a computer, cellphone, PDA, or any other equipment connected to the Web to deliver voice with simultaneous file transfer and instant messagover thethe Internet. It is different from "voice over Internet protocol" (VOIP) networks because Skype is a peer-to-peer system. It creates a computer-to-computer connection over the internet between the Skype users. This connection does not require a central network and therefore there is no middleman, so calls are free.
Check out Skype