From Demo: Truphone works on mobile phones that have WiFi and can route your calls over the data network instead of your cellular connection. Pretty useful for saving money, especially for international roaming, when calls cost a fortune.
The big news is that the company has managed to port Truphone to the iPhone. So now you can make really good use of that WiFi radio in it. In the demo, the presenter showed a call from a phone with no SIM card in it.