WhatsApp is a direct iPhone-2-iPhone messenger chat application. WhatsApp allows you to carry on a conversation in real time with your contacts without having to pay for SMS.

In addition to chat there is a “Status” feature, thanks to which you and your friends will always know what’s up and how to best get in touch.

And with iPhone Push Notifications, WhatsApp can even let you know when your contact’s status changes and what they are up to.

WhatsApp only works among users who have the application, so after you download WhatsApp, make sure to use the “Tell a Friend” feature to share this App with the friends and contacts in your address book.

WhatsApp on iPhone now supports Facebook Connect. Share your mobile status with your Facebook friends and Address Book contacts!

Pros

• Able to use Status functionality of WhatsApp to inform your contacts about your mobile availability
• WhatsApp automatically works with your existing iPhone Contacts. There is no hassle of adding friends, creating accounts, or memorizing PINs and login names.
• No sign up is necessary! Simply download WhatsApp to your iPhone or BlackBerry device and it will automatically work. (Anyone can test this out? iPhone to BlackBerry)
• Super fast text sending user interface with fantastic wallpaper customization support!

Cons

• WhatsApp do not support iPod Touch devices due to their limited network functionality.
• WhatsApp is currently not supported in Malaysia, India, Indonesia, and Thailand.
• Not able to use this app as social networking tool as it’s hard to give mobile numbers to strangers as you can’t block SMS advertising Spam from overseas.

WhatsApp Inc Web Site
WhatsApp Messenger Support