Chathead notifications is the feature that made Facebook Messenger more popular and is probably the most-loved feature of the app. Today, the feature has been implemented in WhatsApp unofficially using an app called Seebye Chatheads. The app has been developed by XDA forum member seebye and is currently in beta stage. At this moment it does implement the Chathead notifications for your WhatsApp messages but it does need a lot of improvement to make the app popular among the masses.
This app requires root access since it access data from another app (WhatsApp in this case), tough luck for people who are afraid of rooting. Though WhatsApp gives you popups for new messages, Chat heads are more user-friendly and provide easy access to conversations. The app also gives a floating notification for you to start a conversation while you might be busy surfing the web or checking emails or whatever it might be.
The app itself isn’t so attractive, but it does the simple job of popping out chatheads on your device. Long-press on the chathead to hide it or you can drop it down as you do in Facebook Messenger. Apart from this, there is nothing much this app provides. The UI of the app surely needs a lot of improvement but considering it is in beta stage right now, you can expect future releases of the app to be more awesome and user-friendly.