WhatsApp is a cross-platorm messaging app for mobile phones. Users can send text messages to contacts as well image, video and audio messages. WhatsApp pull in users from your own address book by their phone number, so you don't have to worry about seeking out contacts or usernames.
The benefits of WhatsApp are that it works over your carrier data plan and is multi-platform and not limited to users on the same type of device. WhatsApp is available now for BlackBerry OS devices and due out in March for BlackBerry 10.