Viber is available in App World (Beta) no voip function yet, so it acts like a cross platform IM app. I have been hearing reports that people who install this and have Whatsapp installed, lose all their contacts in Whatsapp. They still appear but they are only visible via their phone number. Same users reported things going back to normal after they deleted Viber.
Free Viber - Download Viber - Free Apps from BlackBerry App World
I installed it, meh, it's a cross platform IM app. Does ask for your number, connects to your contact list, BBM connected, and I think you can send invites or msgs through BBM as well.(Not sure)