WhatsApp Messenger is a cross-platform mobile messaging app which allows you to exchange messages without having to pay for SMS.
It is available for iPhone, BlackBerry, Android, Windows Phone and Nokia and yes, those phones can all message each other.
Because WhatsApp Messenger uses the same internet data plan that you use for email and web browsing, there is no cost to message and stay in touch with your friends.
In addition to basic messaging users can create groups, send each other unlimited images, video and audio media messages.
First year free then 99c (USD) the year after. No hidden costs. You could send a million messages a day.
As there is no added cost to international email, there is no cost to send WhatsApp messages internationally.