Bubble
This application offers a bubble with the profile image of the contact which has send you a message.
Anywhere
Dashdow WhatsApp overlays every app so you get notified about messages without quit your task.
Popup
The popup appears by click on the bubble and contains the hole messages you got from your contacts.
Free
The free version contains general settings to customize the bubble interactions.
Plus
The plus version adds design settings for personalisation.