So now finally WhatsApp officially (WhatsApp blog) enable users to use them on web browser. However, currently only compatible for Google Chrome browser, and exclude Apple iOS. You may use it at Android, BlackBerry OS, Nokia, Windows Phone.
Download Google Chrome if you don’t have one, http://www.google.com/chrome/, and go to URL below.
You will then see a page with QR code to proceed. Follow the instruction given on the lower part of the web page according to your phone OS and scan the QR code. Then you may enjoy spamming WhatsApp whenever you are, on and off from your phone.