I use a combination of cingular and skype.
Cingular has an excellent domestic and international coverage. Using a quad band GSM phone ensurs you cover the spectrum. Remember to be able to just pop in a sim chip means that the phone has to be "unlocked". I've got unlock codes for nokias and others. This unlocks it from the service provider so you can use your own sim chip. It can be a pain though
I got my wife a blackberry 8700c which has both unlimited international and domestic data for 69bucks a month. While that sounds like a lot my wife doesn't have to carry a laptop on the other side of the world and we can just send an instant message to have a chat or just see if the other one is up.

I've also downloaded a bunch of programs for her that is a currency and airline flight checker and a world clocks. Plus i keep our calendars sycnhed so when i change my calendar it changes on her phone on the other side of the world. 2 pilots takes some planning.
I carry a laptop so what I do when i'm international is keep my phone on. I have the cingular international discounted dialing which is a lower rate but you pay 5 bucks a month. Usually if the person is calling me i'll tell them i'll call them back. Thats where i use skype.
Skype is a good service that is free skype to skype, think aol instant messanger with voice. You can use it also to dial phone numbers for pennies a minute. It works well if you have a laptop you carry.
Vonage has a phone that uses wireless or eithernet internet to make and recieve calls. Works really well is what my friends say but some of them don't bring a laptop. So it really depends on what you do.
Hope this helps. Sorry it's long winded but I've tried a variety of stuff.