xScope is a wonderful utility that runs on OS X. It provides a set of tools that includes letting you measure what's on screen, set up vertical and horizontal lines for alignment and magnify parts of the user interface you are working on.

I often use it to zoom in to check colors as well as alignment as I'm developing on the simulator.

You can buy xScope from the Mac App Store. There's also a trial version available.

Your feedback is valuable: Do you want more nuggets like this?   Yes   or   No