altaibayar tseveenbayar has VisualRuler (iTunes Link) for the iPhone, iPod Touch, iPad in the iTunes App Store for FREE (regularly $1.99) for a limited time.

Need to measure the size of the object but do not have a ruler? VisualRuler calculates the size of objects in the image using computer vision techniques . You will need to use a card as a reference object. Everything is done offline on your phone.

Because all cards (credit card, library card, public transport card, ID card…) have unified side lengths they can be used as a reference rectangle to calculate sizes of objects in the plane.