WhatsApp Inc. has their WhatsApp Messenger (iTunes Link) in the iTunes App Store for FREE (regularly 99-cents) for a limited time.

WhatsApp messenger is a cross platform smartphone messenger currently available for iPhone, Android, BlackBerry and Nokia phones. The application utilizes push notifications to instantly get messages from friends, colleagues and family. Switch from SMS to WhatsApp to send and receive messages, pictures, audio notes, and video messages at no cost. All features are included without the need for extra in-application purchases.