WhatsApp Messenger is a smartphone messaging app allowing you to exchange messages with your friends and contacts without having to pay for SMS. The App is cross-platform friendly, and is currently available for the iPhone and BlackBerry.

Pros:

• Works with existing iPhone Contacts. Adding friends, creating accounts, and/or memorizing PINs and login names is not necessary
• No sign up required! Simply download WhatsApp to your iPhone or BlackBerry device and it will automatically work
• Super fast text sending user interface equipped with wallpaper customization support

Cons:

• Only works among users who have the app installed on their phone
• iPod Touch device not supported due to limited network functionality
• Not able to use this app as social networking tool. For example, it’s difficult to give mobile numbers to strangers as you can’t block SMS advertising. Oversea Spam is also a concern

The App costs .99 cents on the iTunes App Store and $1.99/year on Blackberry AppWorld