This Python program will help determine distances in images with a scale bar, like those taken from a microscope. First, draw a line along the scale bar, enter the distance, then draw other lines.
Here it is in action, measuring the distance of a nanowire transistor. Yes, the microscope has a ruler tool built in, but this works on any computer that can run Python.
Another tip for Mac users, use Platypus to make it into a droplet!