Rebtel: Cheap and Free Calls
Rebtel is likely the best calling app out. It integrates seamlessly with the Android contact manager and the built-in dial pad so you don't have to explicitly fire up the app to get things done, just dial away like normal. Best of all is that the app makes absolutely free calls to anyone with the app installed and to over 50 countries. Unlike most of the included apps here, Rebtel doesn't need a wifi connection to work. It's compatible with a 3G or even basic CDMA connection.