I am entering this project into the UP contest so if you think it is at all neat/delicious/gets you thinking then please give me a vote and help me, help you with more awesome projects =]
What I think is so enticing about this design is the wide applicability. It allows expression of ideas and creativity in normally unused spaces. The real challenge is what's the biggest glass window can you find? This also has other applications as it could clean windows, or survey the sides of buildings, draw just on floors, etc.
The advantage of this design over a polargraph or other drawing method is that it is really easy to set up and draw on any size of window. Later in the 'ible you will find the processing program I wrote for this, which takes input about your window size and robot specs and lets you to draw what you would like the robot to mark on your window; with a touch of a button it turns it into arduino code that fits in the arduino program I have supplied, so it can be easily uploaded to the robot. Then you just put the robot on the wall and watch it go to work! The autonomous nature of this design makes it free of wires while also not relying on any sort of wireless connection to do its work.
I did this project with scrap VEX metal and motors I had lying around, but don't be discouraged if you don't have those!!! It will work with any 3 wire motors (and others as well but may take minor changes to code) and whatever materials you have available (wood, metal, plastic, etc). DO NOT be discourage, you can do this project! I always challenge myself to build with what I have and this project cost me a total of $20 so really not an investment.