To suck people into downloading their free app, Vonage is giving away 15 minutes' free international calling. Thereafter, you can pre-pay for bulk minutes, with the approximate cost being $2 per 15 minutes—billable to your iTunes.
That rate works for 100 countries, but if your chosen country isn't on that list, you can grab an extra 90 countries and talk to them for 15 minutes, for fees between $3 and $10. As the cost of the calls is billed to your iTunes account, you don't need to sign up for yet another billable account you might forget about. [iTunes via 9to5Mac]