I use Skype when I travel on business to call home. A lot of hotels give free internet access and calling from computer to computer is free, so I can talk as long as I want for no charge. I've used Skype from France, Finland and Estonia with no problems.
The only issue I've had with Skype is that there's a bit of a delay, so both people end up talking at the same time. However, once you get used to it, it isn't bad at all. It's also best if both people use headsets, otherwise you can end up hearing your own voice echoing with a delay, and that can be distracting.
I've also got a cell phone with wi-fi and software that works with the Skype network, so if I'm in a hotel with free wi-fi, I can make free calls using my phone. Very cool.
-Rob