Viber is an extremely popular application similar to Tango, but better in some ways.
It has more users -> more people to call.
The application is built into your contact list (no application to open).
You can text people using Viber.

Here is the info thats available on the Website of Viber:
Viber is an application for free international calls and text messages to other Viber users using 3G or Wi-Fi. Uses your existing contact list - check out which of your phone contacts and friends is already on Viber so that you can call and text them for free.