The Face Proximity IR Sensor is what smart phones use to detect when the phone is next to a person's face during a call. They are used to disable the touch screen during a call to prevent accidental commands from contact between a caller's face and the screen.
These sensors (TMD27713) are great for detecting objects at very short distances, especially if the application requires for tiny sensors. If black tape is used, they also work great as line sensors.
The sensors are mounted on a breakout board for easy setup.