I just downloaded WhatsApp Messenger on my iPhone.
It is a Smartphone Messenger that replaces SMS. This app even lets me send pictures, video and other multimedia!
WhatsApp Messenger is available for iPhone, Android, Nokia, BlackBerry and Windows Phone. There's no PIN or username to remember - it works just like SMS but uses your Internet data plan.
Get it now from http://www.whatsapp.com/download/
and say good-bye to SMS.