Fring (App Store link) is a new app for the iPhone and iPod touch that supports free voice over IP (VoIP) telephony. 

Fring lets you use voice and text communications with a variety of services, including fring, Skype, MSN, Twitter, Google Talk and AIM. On the iPhone and touch, service is available using your WiFi connection. 

fring lets you see who’s online before dialing with contact availability indicators, engage in multiple conversations simultaneously via fring’s chat, send & receive files with file transfer capabilities and use  fringAdd-ons like facebook, Gmail Notifier, Vtap Video Streaming, Orkut Social Network, Yandex Push Email and other apps tapping into the fringAPI.

Fring site