One of the features promised with the Facebook Messenger for iPhone update a few weeks ago was Voice-Over IP calling (Download it here). Facebook promised that in the coming weeks you'd be able to call your friends over a cellular or Wi-Fi data connection instead of using up your plan minutes. The feature is finally rolling out now.
We tested voice calling over our office Wi-Fi ad over straight data, and it worked perfectly. Hell, we even called AndrewL up in Toronto from New York without a hitch.
To use the new VOIP, click on one of your friend's profiles as if to initiate a message, then click the little information button in the top right corner, and then from the following screen click "Free Call." But remember, while the calls don't use your plan minutes, they're not necessarily "free." If you're not on Wi-Fi, regular data usage rates apply.
But what's pretty awesome is that you don't need a data plan, minutes, or even a phone at all. As long as you're running a device with iOS 4.3 or higher—an iPad or iPod touch for example—you can call over Wi-Fi. [iTunes]