Cost: NOT Free
WhatsApp is a mobile messaging app that works on several platforms. It allows you to send free chat message from your mobile phone to other mobiles which also have the WhatsApp application installed. The app costs €0.99 to download.
Currently works on the iPhone, Blackberry, Android and Nokia platforms. It does NOT work on the iPod Touch or iPad. WhatsApp for Android 2.x and Nokia Symbian S60 is in beta.
As well as chat messages, you can also send images, video and audio messages. WhatsApp confirms that messages have been delivered by marking them with a little green tick. This app comes in several languages too, including English, Dutch, Finnish, French, German, Italian, Korean, Russian, Spanish, Swedish and Chinese.