Why would you ever futz around with a bubble level, a yardstick, and a pencil when you can draw a straight line with a LASER? <Inception horn>
This laser level from Black & Decker is only $11 today, and can rotate 360 degrees once attached to a wall to project a straight line across the room. It’s not self-leveling like some more expensive lasers, but it does have two backlit bubble levels to help you achieve perfectly horizontal and vertical lines.