Fring announced an early release of their Voice Over IP (VOIP) application that allows you to make telephone calls over the iPhone's Wifi connection. Besides VOIP, it is also a multi-client instant messaging client with support for Skype, MSN, Google Talk, ICQ, Twitter, Yahoo! & AIM.
Bringing PC-benefits to the mobile, fring will empower you with mobility and availability as never before, integrating all contacts in to one, searchable buddy list, allowing you to see whos online before dialing with contact availability indicators, engage in multiple conversations simultaneously, send & receive files and more.
Fring has also provided a Youtube video demonstrating the Fring application on the iPhone. At present, it is only available to Jailbroken iPhones. Download instructions available here.