instaMeasure lets you make estimations of measurements of flat surfaces, or objects on flat surfaces, with your cellphone camera.
Simply put a coin of your choice (penny, nickel, dime, quarter) on the surface, then hold your phone above it with the screen facing up.
Position the camera so that the coin is on the screen, then with your finger touch the coin on the screen.
If the coin was found, a blue circle will appear around it, and two rulers will appear on the screen (one with inches, one with centimeters). These rulers correspond to the size of an inch or centimeter on the flat surface.
Works best when the phone is completely still. Use only for estimation.