Tin Can, created by Perceptive Development, is an iPhone application that allows you to transmit text from one iPhone to another iPhone without using a network connection.
There are a number of ways to get data into and out of the iPhone, such as Wi-Fi or Bluetooth. Tin Can uses sound to send messages between the phones.
With Tin Can, you can type a note on your iPhone, then put your iPhone near someone else's iPhone (also running Tin Can) and transmit your note to their iPhone. You can also forward these messages along further by sending them via email.