Am using Viber desktop client just because of the ease of use. WhatsApp have a web version which is not that easy to use. Now WhatsApp released a desktop client for the Windows and Mac computers.
You can download the application (62MB) and install it on you PC or Mac. When you open it, just like WhatsApp web, you will have to scan the QR code using WhatsApp on your mobile.
If you like to get the beta updates for the application you can select the option from the Help menu.
Unlike Viber it is not an independent application, If you want to use the Desktop application your phone need to be connected to the internet
WhatsApp desktop client is compatible with Windows 8 or higher version of Windows, and for Mac OS you will need version 10.9 or higher.