Plotclock is something of a novelty, guaranteed to put a smile on your face. It uses an Arduino uno to control three R/C servos which guide a white board marker across a surface to write the time of day. When a new time is available the marker picks up the white board eraser and wipes the surface clean before writing the new time. Two of the servos control the X-Y pen movement via a linkage while the third servo lifts the pen at the end of each stroke.
Details of this open-source project are available on Thingiverse where it was posted by a user called Joo. Files for fabricating all the mechanical linkages on a 3D printer and the firmware are available for free. There are also files for a laser cutter if you prefer to make the linkages from 3 mm acrylic. Although the concept is a little impractical for everyday use it’s a terrific example of out of the box thinking. Hats off to Joo!