It wasn't too long ago that I was faced with the task of redesigning a website -- replacing all the images, re-doing an image map and implementing a layout with multiple columns. This task required lots of on-screen measurements, and my method of taking screenshots and measuring them in Photoshop just didn't work. After some searching, I found SnapRuler.
SnapRuler is a simple, lightweight program that helps you measure items on your screen. The app uses a large loupe to help you pinpoint your starting and ending location. It has a snap-to feature that'll jump to the nearest edges or corners, which is handy when you're measuring an object with distinct boundaries. It even snaps to rounded edges, which is something that's difficult to do manually.
The app uses a keyboard shortcut to toggle it on and off, so you can launch it quickly when you need it and shut it down when you're done. Once you measure the object, you can then copy the dimension to the clipboard in CSS, HTML or Objective-C format.
SnapRuler is a basic app that does one thing and does it well. It costs US$9.99 and is available from the Mac App Store.
Hat tip to Stephen for reminding us of this excellent app!