A free VoIP application called Fring has been made available for Apple's iPhone.
Fring lets users make free calls with other Fring users or via Fring using Skype, Skype-out or any SIP provider. Fring combines all your contacts from sundry services within one searchable contact list.
Skype, MSN Messenger, Google Talk, ICQ, Twitter, Yahoo! and AIM are all supported. The software lets users make voice calls over Wi-Fi, and remain connected with chat services using 3G, WiFi, GPRS and Edge. There's also presence indicators included within the implementation.