If you have an iPhone, you have all of the features that can be found in Apple's iPod with the added benefit of being able to make phone calls. But even if you do have an iPhone, call can be expensive depending on the tariff you are using. Viber is a free VoIP app in a similar vein to Skype that can be used to send and receive free phone calls and text messages with other Viber users.
The problem with any such app is that it is reliant on the people you need to get in touch with having the same app installed. This is true of Viber but it does have one big advantage. All that is required is the app itself; there is no configuration required and no need to build up a list of contacts. When you place a call through Viber, you do so using your friends' regular phone number.
There is certainly no denying that when it is working, Viber is a great app and provides a way of saving money on the calls you make. However, there are occasional problems with sound quality as well as random disconnections. It is difficult to complain about such problems when an app is free, but it does make it hard to rely on Viber as your primary means of communication.
For iPod and non-3G iPad users, Viber adds a handy calling facility to an already handy device while providing iPhone users with the ability to save a chunk of cash on voice calls and text messages. With an interface that is almost indistinguishable from the iPhone's usual phone 'app' Viber is a seriously useful tool that's worth trying out.