Want to draw perfectly straight lines without a ruler? Now you can.
Life-Altering Line-Drawing Instrument Design Concept of the Day: Giha Woo’s “Constrained Ball” can be attached to any standard writing instrument to assist in the production of ruler-free lines. In addition, the device has a built-in LCD screen which displays the total length of any line drawn.
As others have noted, this would likely require at least one additional gear to create functional stability, but an interesting concept nonetheless.