Ok, currently this is for iPhone only since the software is only available for iPhone at the moment, but it will soon be coming to Android too. Viber is free app that you can get directly from the Apple app store. As far as making and receiving calls goes Viber is free in the sense that you don't have to buy additional VoIP call credits to use it as is with the case with many other VOIP calling service currently  out there. You only pay for the services that you already pay for and are currently already using. To use Viber you'll need:

- A 3G capable iPhone minimum. Viber uses your 3G data line to make VOIP calls through. It can Also use a Wi-Fi connection to make Viber VOIP calls too meaning that technically it can also be used on an iPod touch and an iPad too although they are not officially supported.

- A copy of Viber installed which is freely available to download from the Apple app store. All you have to do is link your mobile number that you use with your iPhone to your Viber account. Its that simple. Once you've done that every time that somebody dials your Mobile number using the Viber software from their iPhone it will automatically get routed to your mobile number that you use with your iphone. There are no additional credits that you have to buy in order to use this VOIP based service unlike many other VoIP calling service providers.

I can see there being a few potential scenarios where Viber will be very useful to you. For example if...

(No cost impact at all on "all you can eat" 3G data users)

1. ...you are already subscribed to some sort of all you can eat 3G data plan with your mobile provider.

(Completely free to use if you can get access to free Wi-Fi)

2. ...you always have access to a Wi-Fi connection whether at home, at work, at an airport, at a Starbucks, at a Mc Donald's or any other place with accessible Wi-Fi for that matter. I guess technically in this case it could potentially be completely free to use if you constantly have to hang out in places such as Airports, Starbuck's or a Mc Donald's since its free to access Wi-Fi in those places for sure. No doubt there will be many other places too that I've not mentioned here.

(It wouldn't really cost heavy data users anymore then what they're already paying for their service)

3. ...you already use some sort of low cost 3G data plan with your iPhone in order to get access to the internet via a tethered laptop. Using your iPhone to make and receive voice calls via Viber directly from your iPhone would no doubt be far more convenient and less data consuming then using a laptop to make and receive VOIP calls over the internet whilst using your 3G data connection. You could also possibly use Viber whilst you're not actually using the net and on the move to make and receive VOIP calls too.

(Depending on circumstance you could either be getting extremely cheap  or free international calls)

4. ...you often need to stay in contact with other people internationally that you know also own a 3G iPhone.  You wouldn't really be paying anymore then what you're paying already to call people with iphone's internationally when using Viber. Again it could be technically completely free if you have constant access to free Wi-Fi.

If you have a data allowance on your mobile subscription service the use of Viber if using it over your 3G data connection on your iphone (mobile/cell phone) will be included in this data allowance usage.  But if you're already a heavy 3G data user or have an all you can eat 3G data package for your iPhone it wouldn't really matter anyway and would no doubt use up far less data then a laptop to place VoIP calls over the internet. Also when using Viber over 3G I guess the performance could get choppy depending on service coverage and signal strength as with regular cell phone calls.

So apart from being free in all its various technicalities and comparative ways, what other perks or advantages are there of using Viber if you're an iPhone (and soon to be on Android when it finally drops) user? Well the calls are said to be clearer then standard calls.

In testing: At the time I was calling relations in France using Viber. At both ends Viber was being routed through a domestic 4Megabit in home broadband connection. Paying for a  4 Megabit connection on paper at this end really only means you get around 2.4Megabits in practicality when tested by BT. It's even less from behind a firewall at around 1.3Megabits/second.  

But in actual testing despite all the bottlenecks across the broadband connection the sound quality whilst using Viber was unexpectedly excellent. Overall it was consistantly crisp and extremely clear without even so much as a hint of any kind of drop, crackle or pop. Way better then any standard landline or cell phone call. I've not yet tested it over a 3G connection.