iPhone: Viber is a free app that's basically Skype meets Google Voice. It can make regular voice calls from your iPhone, but upon calling another Viber user it will automatically route the call over VOIP and cost you absolutely nothing.
Here's how it works: instead of using your iPhone's phone app, you use Viber instead. When you make a call out of Viber, it checks to see if that phone number is registered with another Viber user. If it is, Viber handles the call over VOIP. If it's not, Viber automatically routes the call through your iPhone's phone app. This way you get free calls to other Viber users and other calls will work just the same as always.
Using Viber is also pretty straightforward, as it's basically a purple version of your iPhone's phone app. Making calls is straightforward, you have access to your address book, and the call quality is decent.
Viber is available right now, for free, in the iTunes App Store.