Hall Effect Sensors are Inexpensive, robust and Dependable, Miniature, and Simple to Use, so you'll Locate Them in Many different machines and Regular devices, from Auto ignitions to keyboards and factory robots to Operate Bicycles.
Here's one very common example you might be using in your computer right now. In a brushless DC motor (used in such things as hard- and floppy-disk drives), you need to be able to sense exactly where the motor is positioned at any time. A Hall Sensor stationed near the rotor (rotating part of the motor) will be able to detect its orientation very precisely by measuring variations in the magnetic field. Linear Hall Sensor like this can also be used to measure speed (for example, to count how fast a wheel or car engine cam or crankshaft is rotating). You'll often find them in electronic speedometers and anemometers (wind-speed meters), where they can be used in a similar way to reed switches.