Henner Zeller wrote Augenmass, a nice in-browser app that let's you quickly grab dimensional measurements from images. To use it:
- Take a picture of what you want to dimension, with a reference ruler in the same dimensional plane.
- Load the picture in the Augenmass app.
- Mark the distances and angular measurements to be measured by drawing lines over the image.
- Enter the correct number for a known measurement (i.e., the ruler or graph paper). All the measurements will be re-scaled to match.
The app features a digital loupe to make precise placement easier. For angular measurement, place the two lines using the same starting point. To start over, just hit reload the page on your browser.
The key to getting good results with this technique is to keep the camera square to the dimensional plane, and use a camera with a good lens. For objects that jut out from the dimensional plane, move your camera further away and zoom in -- that will minimize the distortion and give you a greater depth of focus.
More detailed instructions and the source files are maintained on GitHub.