WhatsApp Messenger is a smartphone messaging app which allows you to exchange messages with your friends and contacts without having to pay for SMS.
WhatsApp Messenger is cross platform and is currently available for iPhone and BlackBerry and iPhones and BlackBerries can message each other too. To send and receive messages, WhatsApp utilizes your existing smartphone internet data plan: 3G/EDGE (or Wi-Fi when available).
In addition to messaging, both iPhone and BlackBerry WhatsApp Messenger users can send each other unlimited images, video and audio media messages.
Download for iPhone from iTunes App Store
Download for BlackBerry