As far as I know, Camera Lucida is the only app available that helps you to draw anything you can take a photo of... on paper, not the screen!
It works by superimposing an image of your choice with a live view of your drawing surface. Yes, it's my own app, but it's still very cool.
Haven't you always wanted to draw a portrait? And like most of us you've said, "I can't draw a straight line!" Well, if you can trace, then you can draw. This program even helps you with shading. Try it!