Like Skype, you can make phone calls to non-oo Voo users (like landlines) if you want to pay a fee.
Since Nimbuzz is a video calling program, you can only video call someone if they are also using the app (be it on their computer or mobile device).
However, their audio calling feature can actually be used with regular phones too, for a small fee.
One neat feature when doing that is that you can see which of your contacts are using Facetime (you can't call someone unless they're also signed up for Facetime).
More » Another similar way to make free HD video calls from your computer is with Nimbuzz.
So long as both users are using the oo Voo app, they can make video calls as often as they like, for Voo was created in 2007 and works with a range of other platforms like Windows Phone and even inside web browsers.