When using international (or national) calls, Skype is the way to go. I have been using it to talk to my parents for a while (on and off) but it is a major player. It is very cheap, costs nothing more than the Internet connection if you use it computer-computer or computer-iphone app.
I have been talking to a hotel room in Thailand from my home here in Sweden and the voice quality was exceptional and even the video quality was pretty decent. Skype is also aquiring Qik which is a video company and might make the video calls even better.
"Skype and Qik share a common purpose of enriching communications with video, and the acquisition of Qik will help to accelerate our leadership in video by adding recording, sharing and storing capabilities to our product portfolio." - Skype blog
While having some trouble with a "global blackout" a while ago it is still a very good product and is based on a peer-to-peer technology which means that there are no centralized servers that the system depends on. Skype uses supernodes that basically are clients with more "power". Skype is also good in couping with NAT and firewalls.
I am going to start using Skype a bit more now and will be trying to integrate Skype with an Asterisk PBX to be able to use the extensions for it and build a VoIP infrastructure with both Skype and SIP. Then also use existing hardware and software for both protocols.