Is there a free calling app for iphone other than skype and Google Voice?

I know pinger only gives you 10 free min. And then you have to "earn" or pay, but is there a completely free app for calling within the US? Thanks!

Added (1). Need one that also assigns you a phone number to receive calls.

Oovoo

Tango, Viber and kik is good for messaging.

Viber
Comm
KakaoTalk
LINE
fring
ICQ
Tango

Check this for online call solutions
http://pakitpoint.blogspot.com/2012/12/free-call-to-any-mobile-and-landline-in.html