Turn your iTouch(2/3) into a wifi phone.
Now that the iTouch(3G) has a microphone, you can effectively use it as a VoIP(wifi) Phone. Siphon is the phone client, Backgrounder will allow it to run in the background, and Insomnia will let it run while the phone is locked. Also the iTouch 2G can be used as a client aslong as you have an iphone headset, sadly the 1G cannot be used.
The combination of Google Voice, Gizmo5, and SipSorcery can provide unlimited calling aslong as you have an internet connection (US only i think).
Configuration instructions are here: Turn Your iTouch into a Wireless Phone! | Ninjas4Hire. I would copy and paste them here however the formatting gets screwed up and it looks like a chaotic mess.