If you make a lot of international calls, the service promoted by Jajah may be of interest to you. You sign up with their service, and when you want to make an international call, you punch the number in on their made-for-iPhone website, and wait for them to call you.
- it's a web app, so you don't need to jailbreak or anything
- for folks based in the U.S. and Europe, but you can call to 122 countries
- it makes no sense to use for domestic calls, since it uses your minutes anyway
- they charge you starting at 2 or 3 cents per minute for international calls, rates are available
- you should verify that it is a lot cheaper than what AT&T would charge you
- they use VOIP, aka voice-over-IP, which is basically telephone calls that use the internet.