HC-SR04 Ultrasonic Sonar Distance Sensor

HC-SR04 Ultrasonic Sonar Distance Sensors work at about 2cm to 400cm. You will get you the best results at 10cm-250cm.

These sensors are fast, fairly easy to use, and low cost. They do require powering from 5V for best results: connect GND (ground) and VCC to 5V power. While the Trig signal can be 3V or 5V, the 'return' Echo signal is 5V logic.

More information is available on the website of Adafruit and Sparkfun.