WhatsApp is one of the widely used instant-messaging application that lets smartphone users to send free texts and media messages including photos, videos and audio to other smartphone users. WhatsApp is now officially available for all windows phone 7.5 users.You can download WhatsApp Messenger For Windows Phone 7.5 from below link
Quote From WhatsApp website
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.
Download WhatsApp for Windows Phone 7 from here