Over the last view days I have been playing with Ping! a currently free instant messaging service for iPhone owners that makes use of the Apple Notifications service.
Ping is iphone-to-iphone messaging that also works with the iPod Touch. Ping! works with push notifications so that messages can be delivered anywhere in the world, instantly. (Click the screen shot for a larger view).
The experience is simple and familiar but does not incur additional carrier charges, like text messaging would. I have been using it with family and work colleagues with great success. I have had the odd duplicate message but all in all the application (free) and service works very well.
One bit of advice is to select your username carefully as there is currently no means to change usernames later. As you build up your Ping! contacts you can attach their Ping! ID to your address book entry making the service an attractive alternative to SMS messaging between iPhone users.
All you need is an iPhone or iPod Touch with the latest 3.0 operating system. Obviously Ping! will only work on the iPod Touch when connected to a wifi network. I spoke to the developer Gary Fung who confirmed the initial release service is free but they plan to charge at a later date.
It is definitely worth a try, download it from the Apps Store (Apps Store Link).