Viber is an iPhone application (Android and Blackberry versions coming soon!) that lets you make free phone calls to other iPhone users that have Viber installed.
When you use Viber, your phone calls to any other Viber user are free, and the sound quality is much better than a regular call. You can call any Viber user, anywhere in the world, for free. All Viber features are 100% FREE and do not require any additional "in application" purchase.