The Structure Sensor is a 3D sensor that can go on to the back of an iPad and turn your iPad into a pretty powerful device.
With the 3D scanner it can measure spaces pretty accurately, finding out the width of a wall, the height of a window and much more. It's actually the perfect companion for DIYers and those who work in work in construction.
It connects to the iPad via the lighting connection but can also connect to other devices via USB.
The SDK is also available for developers to take advantage of the sensor and use it to bring even more functionality.
It retails for $380 which is pretty expensive, so it's not aimed at your average consumer.