http://www.viber.com/

Let’s you call other iphone users having the viber app. It uses your wifi or 3g connection – which practially means it’s free! Android app in development. Free SMS is planned.

VOIP is not new. Skype has been around for ages, but this is directly marketed on iphones – phones being the primary target. And the fact that no registration is needed, besides your own telefon number, makes getting started easy!

Kudos!