WhatsApp for Desktop is application for PC, MAC and Linux which will connect to your Whatsapp and run it locally on your desktop. Its a extension of Whatsapp for Web, you dont need to keep browser open any more, just open this app and start using it. Currently its not supported for iOS device as it fundamentally using Whatsapp for Web APIs.
Its quite easy to install and use. The app is open sources so you can have a look at the source code if you want.
Try it out, you can download the same from http://whatsapp-desktop.com/#download