Every so often I need to measure data from a picture or pdf file. I keep searching for the perfect onscreen set of rulers. I found one a while ago that allowed me to first set up the scale with a few mouse clicks and then do the measurements. Alas, I have lost track of that app.
However, this RealBASIC app seems to be quit up for doing the job, and better still it runs equally well on Windows as on Apple. (RealBASIC should even run on Linux, no?) Check it out at: http://www.omnidea.it/en/software/rulers/index.html