Fring allows you to chat and interact with others using a variety of networks including Skype, MSN, GoogleTalk, AIM, Yahoo, Twitter, and ICQ. Also also brings VoIP using the iPhone’s Wi-Fi connection.

Fring was previously only available for jailbroken iphones but is now available in the Apps Store for free. (Apps Store link) 

Some of the features of Fring are:

• Can call directly using iPhone to another person with iPhone running Fring
• Call MSN or Skype users that are using their computer
• Call using Skype account, which allows you to have your own Skype number and at a discount rate for international calls
• Receive phone calls using Skype with your own custom Skype number
• Basic chat functions with most chat platforms, such as AIM, Yahoo and ICQ