IR Proximity Sensor
This combination of an infrared LED and infrared detector can be used with external circuitry to detect proximity to a surface or the amount of reflection off a surface. An example of this could be detecting a black line on a white surface in a line-following robot.
Please note that this is not a "plug and play" device, and requires a fair amount of circuitry in order to generate a reliable output. The range can be adjusted by a variable resistor in the circuit, normally from 1mm ~ 25mm. There are two example circuits that can be found in the product's images.
Click here to download the datasheet for the sensor.