Straight to the point:
Viber, by Viber Media, will change the way you place calls to your friends, and even more if they also have Viber installed.

As I usually say, Viber is to replace your native “green phone” that you use to make calls on your iPhone. Just take it out and replace it with the “purple phone”.
The idea of this amazing app is to behave exactly as if it were the native one, with a twist: you’ll be placing free calls.
The interface is pretty much the same as the native one (see image above), and whenever possible it will try to place an IP call (over the internet, free). If not possible, it falls back to the GSM, and places the call anyway (carrier charges apply).

As the interface is familiar, there is no adaptation phase, you just install it and continue using it normally.
We have placed lots of calls via IP (commonly called VoIP – Voice Over IP) and the quality is just perfect, when you have the recommended bandwidth available.
One thing different you’ll notice is in the Contacts tab. some of your contacts may have a Viber flag.

This means that contact has Viber installed and if you try to call them, it will probably be a VoIP call, if both parties are in a WiFi zone. And that means FREE.
You also can send messages (SMS) using Viber and, once again, if in a WiFi zone, free.

This app is awesome and it’s a must for every iPhone (it will work on iPad, with the “2x magic”), so you better install it right away.
Viber is free ever since it was released, so spread the word. The more of your friends that install this, the less you pay on calls and messages.
Viber is also available for Android systems, so if you have friends on the other side of the Force, make sure they install it as well, and they will become available to you for free.
Viber – Free Phone Calls & Text on the AppStore