Viber : Free Messages & Calls
Viber allows you to text, call, and send photos and video messages worldwide for free to over 200 million users using 3G or Wifi. Viber is available for Android, iPhone, Blackberry, Windows Phone and many more devices and platforms. On Viber, your phone number is your ID. The app syncs with your mobile contact list, automatically detecting which of your contacts have Viber.