From Forum Nokia Wiki

fring™ is a mobile application that allows you to make mobile calls and send instant messages (chat) to other fring users and PC-based IM services such as Skype, Google Talk, MSN Messenger & SIP at no extra cost beyond your existing data plan. fring calls cost nothing extra because they utilize GPRS, 3G and Wi-Fi mobile Internet connections.

http://www.fring.com/