WhatsApp Messenger is a cross-platform mobile messaging app which allows you to exchange messages without having to pay for SMS. WhatsApp Messenger is available for iPhone, BlackBerry, Android 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.
"All of your chats in one app
Beeper is fast. A single app to chat on iMessage, WhatsApp, and 13 other networks. You can search, snooze, or archive messages. And with a unified inbox, you'll never miss a message again."