WhatsApp Inc. has their WhatsApp Messenger (iTunes Link) for the iPhone and iPod Touch 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 exchange messages, pictures, audio notes and video messages with WhatsApp users at no cost. All features are included without the need for extra in-application purchases.