SMS are one of the most expensive forms of data transfer around, aprox £750 per MB according to one finding (compared to say NASA who pay £61 per MB to receive data from the Hubble Space Telescope). So it's hard not to like this application (despite a name that reminds me of those horrible "Whats Uuuuup" adverts) which lets iPhone users send messages to each other for free.

The nice thing about this app is using push technology it acts pretty much as an SMS, but feels a bit more like instant messaging as there's isn't the (cost based) need to put every thing in one message, so it's more conversational.

It requires both parties to have the free WhatsApp iPhone application installed first, and then users can send as many messages as they want for free. At least it's free if your data plan is unlimited, which for most UK users it will be - though overseas you'll still get charged. A Blackberry version is in Beta.