A free program for Android, by Angel Koh.

Tracer Lightbox is a convenient application that makes it easy to create art. You can choose any image from the device’s gallery or take a photo from the camera, and then you can place a tracing paper on the top of your device.

The app will serve as your drawing table, and, using a tracing paper and a pen, you will be able to draw an image precisely. You can adjust the brightness of the lightbox to suit your needs. Paint, sketch, or trace any image you want! The app has many templates for you to choose from, and you can also look up reference photos online.