See it here at the Nimbuzz blog at http://blog.nimbuzz.com/2009/01/12/free-calls-chat-and-messaging-with-nimbuzz-on-orkut/.
I checked it out, and I think they got me on this greeting: "Deixe seus amigos te ligarem de graça direto do seu perfil do Orkut! - Your friends love you! Let them call you for free!"
Nimbuzz also works on your cell phone. No kidding.