WhatsApp? Nothing, what's app with you?

Keeping in touch has never been easier. Modern smartphones have so many ways to communicate it's sometimes hard to know which one to use. SMS is great for keeping in touch with any mobile phone user, but costs per message unless you shell out for an "unlimited" plan. Instant messaging is great, as long as both parties can get online. And of course, managing different sets of contacts can be a pain.

Wouldn't it be cool if there were a way of sending a message to someone and, if they were offline, having it sent as an SMS message, but if they were online to appear as an instant message? I think so, and so do WhatsApp Inc., developers of WhatsApp, a messaging service for iPhone, Blackberry, and (soon) Android.

WhatsApp is a beautiful amalgamation of synchronise and asynchronous messaging options. The iPhone version presents a UI that bears and uncanny resemblance to the native SMS interface. It uses your existing iPhone contacts list instead of having a separate buddy list, and in its simplest form can be used as a drop-in replacement for SMS for those contacts that use it.

Of course, if that were the extent of it, then you may as well use normal SMS, right? But it's not. For a start, all messages are free and use the data connection rather than the GSM/UMTS connection for sending messages via a central server. If the recipient is offline (i.e. not using WhatsApp at that precise moment) they will receive a message via the iPhone notifications system, just like an SMS. They can then start up WhatsApp and continue the conversation in real time just like IM.

The central server concept works very well. It has been used to ensure that WhatsApp messages are even more reliable than SMS messages. Recipient has no data connection? They can still retrieve pending messages as soon as they do. There is even a handy UI hint for the sender. One little tick by a message indicates receipt by the WhatsApp server, and two little ticks indicate successful delivery to the recipient.

And that's not all! Sending of pictures, sound clips and movies (3GS required) is also supported and, like the text messages, free of charge. There is also an IM style "status" message that can be configured as the user wishes. A nice feature is the setting up of timed statuses. Create an "Out to lunch" status that falls back to "Available" after an hour, and it's saved for quick selection later.

A full set of options is presented to the user so they can configure it how they prefer. Hiding or showing a preview of the message, automatic saving of media to the film roll and even setting of background image is quick and easy. It also supports a landscape mode for us two-thumb typists.

One particularly nice addition is the ability to email chat logs from within the app so they can be archives off-device. Clearing the logs to keep the app tidy doesn't mean losing any history.

All good so far. What's the catch? Well, obviously your contacts have to be running some version of it to receive your messages, and it costs 59p to install. I think that's it. This is a well written, solid app that provides a great way of merging IM and SMS in one seamless interface.