A few weeks ago I was in Paris for Le Web and I think I figured out the best way to call while roaming internationally. There are cheaper solutions out there but this is the best hack for the buck in my opinion.
ATT does a 800mb roaming plan for $120. That puts each meg at $0.15. An average Skype call uses slightly less then 1 meg per minute, so even with the Skype out cost you are under $0.20 cents per minute while roaming on the cellular network. I was worried that the call quality might not be great but in the 4 countries I tired it in (Romania, UK, France and Italy) it was flawless.
You can basically use any VOIP client, I like using Skype because it just seems to work and I have programmed it to show my cell phone caller ID when I call out using it. While this does create call backs to my cell phone I find that so many people screen for caller ID that if I call through anything else I never get through.