Over the past couple of years, every now and then I see someone who has figured out how to control the little piezo electric print heads on print cartridges. This technology is pretty interesting. Basically, certain types of inkjet printer cartridges have the nozzles built right into the plastic housing of the cartridge. The main bulk of the printer is dedicated to moving the cartridge around. Something along the lines of 18V applied to the right pins, cause little droplets of ink to shoot out. If done with the right timing, an image appears.
NicholasCLewis has created a product called the InkShield. This Arduino compatible board can control an HP C6602 type inkjet cartridge. Just start the code up and wave the inkjet cartridge over some paper (or something else) and viola, your design is printed onto the object! The InkShield has circuitry on board to convert the 9-12V input to the 18.2 volts needed to run the cartridge. No wonder these things are so expensive!
Here is the kickstarter, check out the video.
The author over at SpritesMods.com has also put up a project entitled "Electronic Stamp". The concept here is along the same lines, but the resulting machine is more like a magic fairy-wand-printer.
Here is the link
This project gets into the nitty-gritty of the technical issues surrounding how to control an HP print head. Since the above page is not meant to sell a product, the author provides lots of information and a circuit diagram. They can even print onto the foam on top of a beer!