This is a cable driven drawing machine I constructed for the Telus Spark Science Centre in Calgary, Alberta. More information and construction details are on my blog at: http://dbynoe.blogspot.ca/2013/12/four-cable-drawing-machine.html
Each of the four stepper motors is connected to a pulley and string. By winding in or letting out string on each corner the computer can position the draw head anywhere in three dimensional space, or at least within its work envelope of roughly 590x590x220mm.
The machine is controlled with LinuxCNC using a custom kinematics module and python script written by Kevin Loney. Its input files are standard g-code, the same format used for controlling a CNC mill, all the weird coordinate transformation math is handled by the kinematics wrapper.
Accuracy is pretty good, theoretically it can position itself to about 0.1mm, this breaks down near the edges of the draw area as the cables sag, leverage, and a variety of other interesting physics effects take hold.