Ruler is a free Android app developed by Babak Mozaffari that displays a ruler on your smartphone screen to measure small items. This app is a handy tool for those who frequently need to measure small objects but don't have a physical ruler on hand. With this app, you can easily switch between inches and millimeters, and there's even a backlight feature that allows you to see through paper.

The interface of Ruler is straightforward and user-friendly. The ruler is displayed horizontally along the bottom of the screen, and the measurement units can be easily changed with a single tap. The backlight feature can be turned on and off with a simple swipe. Overall, Ruler is a simple and useful app that does exactly what it claims to do.