Analog Feedback Micro Servo - Metal Gear

It looks like a micro servo, it acts like a micro servo, but it's more than just a micro servo!
Adafruit got a factory to custom-make these quality metal-geared 'micro' sized hobby servos with a twist - the feedback (potentiometer wiper) line is brought out to a fourth white wire. You can read this wire with an analog input such as those on an Arduino, to get the servo's position. That information can be used in robotics to improve stability or even allow 'recording' of servo motion.
Note that the default servo pulse widths (usually 1ms to 2ms) may not give you a full 180 degrees of motion. In that case, check if you can set your servo controller to custom pulse lengths and try 0.75ms to 2.25ms. You can try shorter/longer pulses but be aware that if you go too far you could break your servo!
Check out this example Arduino sketch which demonstrates how to record motion data to the EEPROM and then play it back when a button is pressed. Comes with a range of horns as seen above.
Technical Details

Dimensions: 11.58mm / 0.45" x 22.75mm / 0.89" x 29.19mm / 1.14"
Max Dimensions: 11.58mm / 0.45" x 32.74mm / 1.28" x 31.19mm / 1.22"
Wire length (at least): 131.67mm / 5.18" long
Torque: 25 oz*in / 1.8 kg*cm @6V
Speed: 0.1 sec/60° @6V
Weight: 15.81 grams
Dual ball bearing
Metal gearbox
Approx 120 degrees of rotation (500 to 2500 microseconds)
Spline Count: 22