Nice NXT based printer from Leon Overweel.
Leon creates a design by first loading up an image on computer, converting it to black and white and then into 1's and 0's. That array is then copied into a RobotC script that puts down the pen for the 1's and lifts the pen for the 0's as it scans along. Nice and smooth, and great resolution of 50px by 50 px.
Leon has a comprehensive post with all the fine details.
(Thanks to Joey Kelly for pointing me to it!)