WhatsApp Messenger is a smartphone messenger available for Nokia, iPhone, Blackberry, and Android phones. WhatsApp uses your 3G or WiFi (when available) to message with friends and family.
The V1.6 update for Windows Phone is a bigger update with a bunch of improvements.
Here are the more important ones:
- Message Forwarding
- Unread Message Counter
- Ability to add new/unrecognized users
- Quicker download of pics on EDGE networks
For a more complete list click here.