Check out this video of a home-brew printer designed and built entirely by some guy on the B3ta forum.
He rigged together a sensor, a USB interface with a wiring board, and a series of analog motor electronics to put together the device that can legibly spell out whatever he wants, in this case the existential message "HELLO WORLD," although since it uses a printer driver to put ink on paper, it should also spell out "LOW INK LEVEL, PLEASE REPLACE AGAIN AND AGAIN AND AGAIN AND AGAIN."
For more detailed information, check out a list of FAQs with the inventor after the jump.
FAQ: Track name? "Christopher and Raphael just popper Shinichi Osawa distortion disco edit"
FAQ: Are you using some sort of MCU demo board (the black pcb)? Yes.  http://wiring.org.co/hardware/FAQ: DPI? Dots per inch? Approx 75 DPI.
FAQ: How long did this take? Hard to say, maybe three weeks working evenings.
FAQ: Open source, schematics etc. Yes, I'll try to get around to this soon.
FAQ: PPM? Pages per minute? Maybe one is pushing it, I sped things up a little in the video editing to keep it interesting to watch :)
FAQ: Wrote your own driver? Yes, how sad is that!!
FAQ: Mac vs PC abuse. Video not meant as a Mac advert and I'm sorry if the Apple logo offends anyone. I just find them easier to use.
FAQ: Felt tip damage drying up? Yes this is a problem, but one felt tip usually lasts for quite a few pages.
FAQ: Full color version/more colors? Originally I was going to do three-color version, but I had to simplify as only have four Lego motors.