WhatsApp Windows Phone
The popular IM Messenging client WhatsApp appeared in the Windows Phone Marketplace a few days ago.
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, Nokia and Windows Phone devices 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.
WhatsApp for Windows Phone 7 is freeware!