If you use Photoshop for scanning images, you often need to rotate the scanned image by a degree or two to get it lined up straight (placing it straight on the scanner bed can be almost impossible).
I used to use the trial & error method of guessing a rotation amount, trying it out, then adjusting it up or down until I got something that looked right. Eventually, I figured there must be an easier way.
And of course there is: use Photoshop’s Measure tool, which is a sub-menu option on the Eyedropper tool. Measure a horizontal or vertical line using this (the longer the better), then open the Rotate Canvas / Arbitary dialog and you’ll find Photoshop has automatically filled in exactly the right angle for you.