I recently met Talman Marco, the CEO of Viber (above). This startup allows you to use your iPhone to make free international calls over WiFi or 3G to any other Viber user.

"We had more than 1 million downloads in the first few days of launch. Going through passport control I even saw it open on the phone of the woman control officer," said, Mr Marco.

You can download it here:

You can read more about Viber here:

Engadget:

Out of nowhere, Viber has swooped in to crush the dreams of Skype and every other VoIP provider out there, and if these guys can reach critical mass, avid iPhone users will certainly be able to buy a mobile plan with fewer minutes. It works as such: install the free app on your iPhone, provide just your phone number (no "registration" is necessary) and then fire it up. Viber uses your existing contact and favorites list, and you can make Viber or standard voice calls from within the app; naturally, a Viber logo pops up beside any contact who is also a user, making it easy to see who you can VoIP call for free.

Daring Fireball:

Call quality, even over AT&T 3G, is pretty good — far superior to actual phone calls. The poor audio quality of iPhone voice calls in the U.S. is shameful. A call to a friend in the Netherlands sounded great too — a few very brief glitches, but good sound overall and very low latency.