Skype is a proprietary VoIP application that provides a free voice call service (to other Skype users) and a prepaid service for dialling out to fixed or mobile phones anywhere on the planet.

It supports all major OS platforms including Linux and OS X. I've created an account: jduprey. I've yet to try it out. If you already know me, give me a ring via Skype so we can both try it out. Its free!