Android only: Rebtel is known for its cheap international phone service, but recently they've regained our attention with a new Android app that lets you call other Android users in over 50 countries for free.
Not unlike the ever-popular Skype, Rebtel's service is based on rerouting international calls to give you cheap rates. However, if you and your overseas chat buddy are both using Android phones, you can now use Rebtel's Android app to call one another for free, as long as they're in one of Rebtel's supported countries. You'll still use up regular minutes on your phone, but only as if it were a local call, since Rebtel connects your call through a local number.
The system is set up quite nicely: Rebtel works in the background, intercepting long distance calls and routing them through their service. That way, you don't need to use the Rebtel app to make the calls - you can just make them from your regular Android dialler or address book and it will intervene on its own. And, best of all, you aren't limited to Wi-Fi networks - you can use it anytime, anywhere.
Unfortunately, I have not been able to test the actual calling features of the app, since everyone I know lives within a few hundred miles of me, with the exception of one lone iPhone user in Germany. So if you have friends or family overseas, try it out and let us know how it works for you in the comments.