It's not entirely kosher according to Apple (you'll need a jailbroken iPod touch to pull it off), but it could be interesting.

The guys at Lifehacker have posted an outstanding full guide as to how to install a freeware application called SIP-VOIP in order to make the occasional VoIP-based phone call from your iPod touch handset.

What you'll need:

1.) A jailbroken iPod touch.

2.) A Touchpod microphone (retails between US$50 and US$60.

3.) The freeware SIP-VOIP application (full information available here. Users can install the program via the Installer.app component found on many jailbroken iPhones and iPod touches.

Full setup instructions are available here, so take a gander and let us know how it works in the comments or forums.