WhatsApp is now available for desktop in the form of an in-browser web app, compatible for now only with Google Chrome.
Since WhatsApp requires an associated mobile phone number to log in, users need to scan a QR code from the web app using their smartphone to access the desktop version. The online version works in conjunction with Android, BlackBerry, and Windows Phone, but not yet with iOS devices “due to Apple platform limitations”.
Since the WhatsApp is connected through your phone, it only works for as long as your phones does: if your handset loses battery life, the web app disconnects.
The WhatsApp web browser is available here.