WhatsApp Messenger is one of the preferred IM applications for mobile platforms. It is available for free (for the first year) for most mobile OS ( BlackBerry , Android , Windows Phone). But that's not the case for the iOS app. It costs $ 0.99 and you can download from the Apple Store.
However if you don't want to buy it, there is a webpage that allows users to send WhatsApp messages from an iPad or iPhone.