Wow, I just can't get away from this guy. I went on Brickshelf (a LEGO image sharing site), and one of the most recent postings was Gregvader's ellipsograph. An ellipsograph is also called a 'trammel of Archimedes', though it is unclear if these actually go back to Archimedes' day. Anyway, as two sliders go back and forth in their tracks, a pen on the swing arm draws out an ellipse. I've included a couple of animated gifs from Wikipedia so you can see this in action. The Wiki description notes that there is a children's toy called a "Do Nothing Machine" that is basically the same idea, and that reminds me that a LEGO version is described in the Klutz Book of LEGO Crazy Action Contraptions. I've included a version by BentWright and you can see a video showing it in action.