xScope by Iconfactory ($29.99) is one of the most indispensable apps in my Toolkit. I often need a screen ruler to not only measure onscreen elements, but also to make sure design elements and text are all lined up nicely. xScope’s screen ruler mostly stays out of the way (I just have one complaint that it steals your mouse focus if you try to click something near its edges). It’s lightning fast, so it’s easy to close the app and reopen it when you need, or simply hide the ruler and leave xScope running.
xScope also includes a number of other useful applets, including a very nice loupe that displays color information.
I’ve been using xScope for a couple of years now, and it’s high-time I gave it the praise it deserves. This is an app that I would have a heard time working without, so I hope it’s here to stay.