A 3 degrees of freedom arm that knows its position by the variable resistors that form its joints.
The arduino reads this resistors and prints the code by serial, where a python script running inside FreeCAD waits for the data.
I'm impressed with its accuracy because it recreates the real world objects with some kind of precision, in spite of being poorly built.
This photo is an example:
The propeller seen in the first picture:
With a better built arm, I'm sure this can improve enough to be usable.