Skype uses your phone's data connection to make free calls to other Skype users. And unlike Apple's FaceTime video chatting feature that only works on Wi-Fi networks, Skype users can video chat with each other over a 3G connection. Add in low-cost international calling plans, built-in instant messaging functions, and CD-quality voice transmission and you've got this whole communication thing covered from just about every angle.
Filling up your new iPhone? Just hoping to make the one you already have even better? These are the apps you don't want to be without.