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.