Vonage Mobile app brings voice calling to your Facebook friends list
I like the concept of this app, considering that Facebook is usually the best contact list you can have, and not every friend you might want to talk to will share a phone number on a social networking page. The iPhone version of Vonage Mobile supports push notifications, so you can take calls while it's not running, which is handy. It also works as a Facebook chat client, if you'd like to IM people before you call them. Of course, it's not the Phone app, so it's relegated to using an iPhone headset instead of the phone speaker.
You can get the Vonage Mobile app in the iPhone App store or the Android Market. None of my 900-something Facebook friends have it yet, but I think I'll keep it on my phone and see if it catches on.