WhatsApp Messenger is one of my favorite ways to keep in touch, with friends and family, while I am traveling. WhatsApp is a 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! WhatsApp Messenger uses the same internet data plan that you use for email and web browsing, so just make sure you have a good data plan. Of course, you can always connect to WIFI to send messages for free!