I'm not sure if Meazure will measure up to Screen Calipers.
But it's free although the author does request you to consider a donation.
On the very odd occasion that I need to measure an object on screen, I do find this quite useful.
What are the dimensions of that image? How big is this window? Does this layout fit on an NTSC display? What color are these pixels? What does this icon look like close up? How can I capture an arbitrary portion of the screen? These are just a few of the questions that kept coming up as I developed applications and web pages. Various tools are available to answer some of these questions but I could not find any that answered them all or that had a usable interface. So I created Meazure, a program that measures, magnifies and captures the screen by providing a battery of features in a cohesive user interface. Even better, Meazure is free!"http://www.cthing.com/Meazure.asp