[Wyatt] built this smart posterbot a few years back. This robot will draw out any small monochrome bitmap onto poster paper. It can mark out the individual pixels just as a printer might. It works best with strings of characters.
Lacking the gift of girly handwriting, he grafted an inkjet printer onto his Roomba to make posters for him. There have been other bots to do this, my favorite being the graffitiwriter. Of course, [Wyatt]‘s bot is less likely to get you arrested.
An Old inkjet printer
An iRobot Create and Command module
Electronic prototype board or bread board
Several rubber bands
Two 9-pin serial cables with at least one male end
4 PC-Mount DPDT 5v DC relays rated at least 1A
4 2n222 transistors
4 1k 1/4 watt resistors
some wire to make jumpers with (unbraided)
~3′ of 1/8″ Bass wood
Butcher paper (get at least 20′)
Voltage regulators (values vary)
White acrylic paint (optional)
Full instructions here: http://www.instructables.com/id/EM9RWKMF5Y3YTS2/