Skype for Android
The new official Skype app for Android brings free Skype-to-Skype calls over 3G or Wi-Fi connections (Wi-Fi only in North America) to current and new users of the popular consumer VoIP system.
Existing Skype users can access all of the same contacts that they would use on their PC, and either communicate with them via instant messaging or call them directly via Skype. National and international calls meanwhile can be made by purchasing Skype credit. A full list of call tariffs is available via the Skype website and it really does work out remarkably cheap.
Recommended for newer Android devices, you can download this free Skype for Android app from the Android Market or via skype.com/m.