The OctosonarX2 provides a convenient way to interface up to sixteen HC-SR04 ultrasonic distance sensors to the Arduino without using up your pins.
The software library handles the timing of the pulses and records the distance detected for later retrieval by your Arduino code.
The Arduino communicates to the OctosonarX2 over the I2C bus to trigger the sensors and uses a hardware interrupt pin to accurately record the timing of the returned echo pulse.
You can use this for any project where you want to use multiple HC-SR04 sensors. I designed it so I could place two sensors on each side of a rover so that it could detect the angle relative to the wall of a maze without having to move forwards – and have more for the blind spots still remaining at the corners.