What I love about robots is that you can make a thing that makes other things. So meta! It uses gcode draw any picture you like, or you can drive it around like an etch-a-sketch. You can scale this robot up to almost any size. The version in the photograph is my smallest model. I've built three of these now, the largest of which was 3m*2m. This particular robot costs $150 in parts and, if assembled correctly, can be up & running in about 20 minutes.
You can now buy this robot as a kit on the Drawbot's official page.
Step 1: How does it work?
If you know the distance between the steppers and you know the length of the strings then you can calculate the XY position of the pen. If you know the XY position of the pen and the distance between the steppers you can find the length of the strings. From there you can figure out how much the strings have to change to move from one place to another and then you can draw lines and curves.
The rest is boring tricky stuff, nice but not vital. There's some code to make sure you can't draw outside the drawing area, and some code uses trapezoidal motion interpolation to draw accurate pictures faster.