Slider-style linear potentiometer. The resistive element was printed using conductive ABS filament. This design isn't perfect as the values still fluctuate a little bit when adjusting the slider, but usable and a LOT better than my previous designs. Also sub-optimal is that I'm only reliably able to get about 4 bits of resolution out of it.
It's tricky to print useful potentiometers out of currently available conductive ABS filament because of the very high resistance (think megaohms) and the fluctuation in resistance with pressure and bending. The orientation that you print the resistive element matters a lot. Increasing surface area seems to help as do springs to stabilize the pressure.
Here's a video: http://www.youtube.com/watch?v=TaeihVHN1Wo