Analog Linear Hall magnetic module
This module comes in two versions, the digital version (available here) needs to be plugged into a digital pin and the analogue version into an analogue pin of your microcontroller.
This product reacts when they come in contact with magnets. An example can be a door open/close sensor if you have a sensor on the door frame and a magnet on the door. This unit will pick up when the door is open or closed. They are often used on wheels, etc., to count the times or wheel rotation speed.