WhatsApp Messenger is a smartphone messenger available for Android, BlackBerry, iPhone, Windows Phone and Nokia phones. WhatsApp uses your 3G or WiFi (when available) to message with friends and family. Switch from SMS to WhatsApp to send and receive messages, pictures, audio notes, and video messages. First year FREE! ($0.99 USD/year after)


Who's on WhatsApp?
Every WhatsApp account is tied to a single phone number. When you launch the app for the first time, WhatsApp automatically scans existing phone numbers in your address book and extracts the ones using WhatsApp, so you can begin messaging those people right away.

I was surprised by how many of my contacts already used WhatsApp. If you can't find your friend, you can easily shoot them an invitation from within the app. A small, but helpful, additional feature is that it automatically adds new buddies when they join the platform – the app requires read/write permissions for your contact information, and it will run a scan every time you add a contact number.

WhatsApp is based on an open source chat protocol, XMPP, the same one used by Apple iMessage and BlackBerry Messenger, which has been the target of a few successful hacks. For instance, last year security researchers were able to intercept and read WhatsApp messages by sniffing WhatsApp data over insecure Wi-Fi networks.

Fortunately, the current version encrypts your messages so that even if someone captures them in transmission, they won't be able to read them.


WHATSAPP:

• NO HIDDEN COST: Once you and your friends download the application, you can use it to chat as much as you want.  
• NO NEED TO LOG IN/OUT: No more confusion about getting logged off from another computer or device.  
• NO NEED TO ADD BUDDIES: Your Address Book is used to automatically connect you with your contacts.  
• OFFLINE MESSAGES: Even if you miss your push notifications or turn off your phone, WhatsApp will save your messages offline until you retrieve them during the next application use.
• AND MUCH MORE: Share location, Exchange contacts, Custom wallpaper, Custom notification sounds, Landscape mode, Precise message time stamps, Email chat history, Broadcast messages and MMS to many contacts at once and much much more!
• MULTIMEDIA: Send Video, Images, and Voice notes to your friends and contacts.
• GROUP CHAT: Enjoy group conversations with your contacts.
• NO INTERNATIONAL CHARGES: Just like there is no added cost to send an international email, there is no cost to send WhatsApp messages internationally. 
• SAY NO TO PINS AND USERNAMES: Why even bother having to remember yet another PIN or username? WhatsApp works with your phone number, just like SMS would, and integrates flawlessly with your existing phone address book.