PixelSnap

PixelSnap can measure distances and objects on a Mac screen. Firing up with the shortcut Shift ⌘ X, my typical two use cases are:

• Checking the alignment of items on a website. The crosshair lines (indicated red) and the alignment guides (indicated by blue lines) support this well. To keep a horizontal crosshair line while moving on with measuring, press H and you can do the same for vertical lines with V. To enforce a crosshair for the entire screen press Shift while moving the mouse pointer, otherwise PixelSnap will anchor the crosshair lines to the objects near to the mouse pointer . To add an alignment guide, press Shift H or Shift V and then click where the guide should settle. It´s more important for me to see the alignment than to measure the exact distance. ESC will clear all indicators from the screen.
• Snapping an object by dragging over it and letting PixelSnap adjust the selection area to the correct size of the object. Once an object is snapped, the exact dimensions are shown, which can be copied to the clipboard by pressing Enter. It´s possible to take a screenshot of the snapped area by pressing S, or by clicking into the center of the snapped object.

Optionally, PixelSnap can integrate with CleanShot, a companion app that´s powerful for doing screenshots.