I have been using Viber, a free VoIP and messaging service that was recently released on the Android platform. Previously it was also available on the iOS app store where it has been downloaded over 12 million times. Viber is similar to WhatsApp in that it uses your phone number as an identifier and not a user login, and uses data (WiFi, HSPA etc.) rather than cellular connection to make free calls and messages.
In fact, I am already a convert. Find out why in my review of Viber for Android at FoneArena.