The Sharp IR Distance Sensor GP2Y0A41SK0F is a great low cost robotic range-finder. The Distance Sensor will report the presence of an object 4-30cm away from the sensor. The sensors reports this data as an analog value readable by most microcontrollers, like the Arduino. This sensor can plug directly into a RobotGeek Sensor Shield using a Sharp IR to Servo Cable. This cable can also be handy to connect to other systems and controllers (users can cut/splice the wire). If you want to use this...

One of the most favored IR range sensors on the market. Detects distances from 4" to 30" (10cm-80cm). Note: The sensor sold here is actually a Sharp IR GP2Y0A21YK0F. It has the same distance range as a GP2D12 and very similar analog output properties. The GP2D12 is extremely difficult to source currently, so these sensors provide a great alternative. For more information on the minor differences, please refer to the datasheets for each sensor: Datasheets GP2D12 Datasheet GP2Y0A21YK0F...