Skype is a very popular software that allows to make telephone calls over the Internet. Calls to other users of the service are free, while calls to other landlines and mobile phones can be made for a fee. iPhone 2g, 3g ( firmware 2.1, 2.2) , iPod Touch have several apps that support connectivity with Skype and can be used for WiFi.
Fring, a mobile VoIP client, offers Skype out calling along with other clients Yahoo 
Messenger, Live Messenger, Google Talk, ICQ, Twitter and AIM. Download Fring from App Store.
Nimbuzz has a free application which users call, chat and message, for free, with their friends
on virtually all major instant messaging communities and social networks, including Skype, Yahoo! Messenger, Facebook, MySpace, AIM, Windows Live Messenger, Google Talk, ICQ, Jabber, and more. Specifically for the needs of iPhone users, this will be the first complete messaging product available with landscape mode chat. Download Nimbuzz from App Store.
Truphone, also allows users to make and receive Skype calls to its iPhone / iPod Touch 
application, as well as adding support for Twitter, Google Talk, MSN Messenger, and Yahoo Messenger. Download Truphone from App Store.
