The Constrained Ball, designed by Giha Woo, is an amazingly simplistic and fun solution to drawing straight lines without a ruler.
The device works as a mountable restraint of sorts. By slipping the pen inside the Constrained Ball, a user’s naturally pivoting angle when holding a pen allows the ball to control the rolling in only one direction. Also, there is a feature on the device that measures the length the user wants the line to be. The mount is made of rubber to provide a tight grip on the pen.