Do you fancy Blackberry where the users could text other Blackberry users around the world by simply entering the contact's Blackberry PIN? Blackberry use Blackberry Messenger to do that.
Do not own Blackberry device? No worries. WhatsApp is here!
Similiar to Blackberry Messenger technology, WhatsApp uses push system to delivered messages. What the best part here is, the app is available cross-platform! - now iPhone, Blackberry, Symbian & also Android can message each other.
Blackberry Messenger uses PIN system to identify users and the phone, however WhatsApp uses phone number as an ID! Means that, even you do not know your friend's device ID, you just need their contact phone number to get connected to each other. How cool is that!
Download now and use! iPhone and Blackberry version are available via AppStore & AppWorld. The Symbian & Android version are still in beta version, only available at the developer's website.
Not only text, audio, picture, video, and even you can share your location!
There is also another similiar app called Cnectd. But its kinda crap. :P
Here is the link anyway.