Slide Pot - Motorized (10k Linear Taper)Sparkfun
These motorized sliders are very cool. Each is essentially a standard slide pot which is belt-driven by a small motor. The slide contains two separate 10k linear taper potentiometers so that you can use one as servo-feedback in order to read the position of the slider and use the other to control whatever your target is. There is also a touch sense line which is electrically connected directly to the metal slider tab so that you can interface the slider with capacitive touch circuitry.
Motorized potentiometers are useful when you need the ability to jump to preset positions or when you want physical feedback from virtual controllers. There are also a variety of unconventional uses for these potentiometers such as pulsing the motor to provide haptic feedback or just using it as a linear actuator for super light-duty robotics.
Check the related items for plastic knobs that fit these sliders.