A rotary plate that not only rotates but also gradually moves up and down. Might be useful for 3D scanning applications.
See it on YouTube.
Assemble the frame with some parts from old Soviet Konstructor toy (pretty much the same as Erector Set). Assemble printed arms with six M3x12 bolts. Screw 24BYJ48 (or any similar) stepper motor to the frame and connect it to your Arduino board. Drop M4 nuts into nut traps, place a spacer between two nut traps, then fasten this assembly to the frame right above motor shaft. Screw M4 threaded rod into place and put printed table on the top of it.