The perfectionist's screen ruler

Requires OS X 10.8 or newer — Available on the Mac App Store

Set a global hotkey

Use the crosshairs and zoom to make sure what you're working on lines up perfectly.

Measure in pixels or points

Whether you have a Retina Mac or an older display. Get the dimensions of on an area without taking a screenshot.

Copy to clipboard

Find recent measurements in the status bar menu. Your last measurement can be copied to the clipboard automatically.