CamRuler is a measurement app that uses common real-world objects in a photo to measure other objects in the photo, but it's more trouble than it's worth.
When I first saw the app I thought it would be pretty useful -- people might be able to find the dimensions of their kitchen, for example, by taking a picture with a reference object. To be fair, when you can make it work, the app is quite accurate, but the interface keeps anything from being easy.
In my testing I tried a couple of objects within the app that have known measurements: a quarter coin and an 8.5x11 piece of paper. CamRuler has several objects you can use for reference in categories such as U.S. currency and standard paper sizes. You can also create and save reference objects of your own if you know their exact measurements.
I selected a quarter as my reference within the app and placed a quarter on my desk. Then I put a pen next to it and snapped a picture within the app. In the next step, CamRuler has you resizing a circle (using pinch gestures) to match up with the surface area of the quarter. After matching the circle with the quarter, the next step is to match up both ends of a line to the length you're trying to measure, so I resized the line to the length of the pen and CamRuler told me how long it was. I checked with a tape measure afterward, and sure enough, CamRuler correctly measured the pen.
Obviously, when perspective comes into play you're going to get different results, and help files in CamRuler are careful to point out that it can only accurately measure objects that are in the same plane with your reference object. But the problem is not whether the app works, it's how hard it is to use.