Some time ago I found the following youtube video by Ricardo Oliveira of a small NXT plotter that crawls over a whiteboard. It has two strings that can be shortened through coils and with this, it controls its position on the whiteboard.
Here is a screenshot from the video.
The whole idea behind it is as simple as it is intriguing. I really like it. There is a somewhat more detailed video (thanks to Mr. Shiny for sharing the link) here on youtube.
Nevertheless, could someone who has already recreated it and could they share detailed images (or even better a construction plan)?