There is a new VoIP provider for international calls that I'd had a play with this afternoon that could certainly become big very quickly.

www.rebtel.com

You register a phone number and enter the number of a friend overseas. You then get sent phone numbers by SMS that are local calls (ie in the same country). You have to register (US$1 per week) then both dial these numbers and essentially enter a conference call together and can talk for free, the only costs incurred are those for the call to the local access number. NZ numbers are only in Auckland but it's certainly an interestintg concept.