Someone at work broke my favourite mug. Well, the freebie mug I was using. Clearly I needed a new one, and the only logical solution was to build a machine to draw the image of my choice onto a blank mug.
Evil Mad Scientist sell an egg bot kit – it draws a design onto an egg (or other broadly spherical object) using a couple of stepper motors, controlled through an Inkscape plugin. The control board is available separately (the whole project is open source), and is a fairly neat 2-stepper and single-servo controller, connecting to the host PC as a serial port over a USB cable. Hence most of the electronics and software were already taken care of, and I could concentrate on the hardware.