Launch WiFive application and immediately start chatting with those who is using it too within your Wi-Fi network. No Internet needed. Also, there is no need to set up a server. Simple, eh?
But of cause there are some conditions.
In order to receive messages your device should not ignore broadcast UPD packets (as long as I know some devices are doing it, for example HTC)
Your Wi-Fi network should not block UPD packets (as long as I know some network managers are doing it). But it's OK if you are creating Wi-Fi Spot yourself. Google it, it's pretty straightforward.
Also, be aware of some possible message loss.
Have fun with unlimited communication! :)