An update to Viber has hit the App Store today. I was impressed with the original release of this free VoIP client when it came out early last month. I especially liked the voice quality and the notifications. Since the review, Skype with video finally made it to the iPhone and attracted a lot of attention, but I still prefer the audio quality of Viber, and unlike Skype, it doesn't have to be running to get a call notification. Of course, everyone you want to call must be running Viber, but that is the only requirement.
Viber is also coming to BlackBerry phones and Android. I don't think Viber is ever going to threaten Skype, but in some ways, I like it better. If you are a frequent caller, especially if you call internationally, it's worth a look. Viber is voice only, so no video calling.