Skype for iPhone - now avaliable and is FREE [AppStore, Free]

Fring allows you to chat and interact with others on a variety of networks including Skype, MSN, GoogleTalk, AIM, Yahoo, Twitter, ICQ.

In addition, Fring offers voice over ip (VoIP) using the iPhone's WiFi connection. Features listed include:

• VoIP Calls over WiFi
• Instant Messaging
• Integrated dynamic contact list 
with real-time contact availability
• SIP integration
• Multiple Connection types

Fring supports SkypeOut and SIP which allows you to make calls to landline and mobile phones. Available for free via App Store. (link)

via macrumors