A device that can print visual images from your short and long-term memory.
The brain printer does not print brain waves or abstract electrical patterns: rather, when you call to mind an image from your memory it will print that in photographic fashion, as you see it in your minds eye.
For instance, suppose that you and friend visit a foreign land together. When you return, you find yourselves talking about the plaza at the center of town. He remembers yellow window-shades that flanked the building across the square from the coffee shop. You do not.
Upon printing both of your memories ("think of the plaza, as seen from the Cafe, then select 'Print'"), you can see the vibrant colors in his, while you may find yours dark, fuzzy and empty save for one piece of statuary next to a newsstand.
These pictures of course do not confer validity on the events and places being remembered: you would need a camera for that. But they can show you how your memory works, how you process external input, and to what degree you are a visual thinker.
The company that has developed the Brain Printer is in the process of developing an audio-sensory output device that will playback your full experience of an event: the sounds, feelings, and images that create that picture in your mind. This device will not be ready anytime soon, so in the interim we will rely on the existing technologies for experience-translation: words.