Facebook Messenger was updated today with free VoIP calls for everybody. VoIP calls was added to Facebook Messenger in January of 2013 but it was limited to the US, UK and Canada until today. You can call your friends for free over WiFi or using your cellular connection. The features basically works just like FaceTime Audio which comes natively with all iOS 7-devices and Macs running OS X Mavericks 10.9.2, and the UI looks almost exactly like the Phone app in iOS 7.
Facebook Messenger is available for free on App Store