A quick video of the HP Plotter (at actual speed!)
This is a machine I admittedly did not build myself. Indeed, my first "intact and usable" techno-junk find was a small HP pen plotter (7470A). HP engineers figured out how to make the pen and paper move--all you need to do is feed it a list of places to go (an HPGL plot file) and voila!
Alas, pen plotters have been completely replaced by faster, cheaper, and more reliable ink-jet systems. This makes the large E-size plotters (36" wide paper), which were $12-15,000 twenty years ago, available now for ~$100. Man, could HP build'em (these things are tanks)!. Watching these high performance plotters in action is quite thrilling. Servos zip the pen and paper past each other at speeds that are difficult to see, all while the pen up and down movement taps out the rhythm of the plot.
I use the plotters in the classroom to
show off how interesting techno-junk can be
display a cool example of Descartes' x and y idea put to practical use