One of my new favorite apps, Viber. It’s a free download from the app store and allows you to make free voice calls from your iPhone over (wifi and 3g) to anyone else in the world that has Viber installed and running.
The nice thing about this app is how it’s integrated with your contacts. People that have Viber installed will automatically show up and allow you to make a Viber call or a regular voice call over your carrier’s network. No need to create a username and share with your friends. Just install and you are good to go. Check it out!