This great sonar ranger from Devantech offers a wide range from 3cm to 3m. For further range see the SRF08. The SRF04 is interfaced using the one output line to ping the sensor and an input line to read the echo.
This is the SRF08 Ultrasonic ranger made by Devantech. The Ultrasonic detector can detect from ~4 inches to ~30 feet.
This is the SRF05 Ultrasonic ranger made by Devantech. The Ultrasonic detector can detect from ~4 inches to ~13 feet.
The ROBOTIS HN07-I101 Idler Bearing Set provides a passive idler hub on the back side of the servo for building joints using the RX-24F, RX-28 and MX-28 Dynamixel servos.
Mini 3.5mm Flexible Microphone