For the past few days I have been using Fring, a mobile VoIP application for mobile devices. I truly like that it has support not only for its own network, but also for calls and chat using Skype, MSN, Google Talk and SIP.
The application, however, is still in advanced beta stage, so some parts of usability and especially stability are still issues. If you have a S60 device with WLAN (such as a N93) or unlimited 3G, I'd say it's almost a no-brainer, especially since Skype hasn't released its version for S60 yet to the general public (other than owners of the X-series).
Now I wonder, who will have the idea of joining this with a Fonera? That would be something.