http://www.iconico.com/caliper/

This site gives access to four measurement tools.
Note that the demo version give FULL, permanent, (limited) usage---you do not have to buy!
Brian

I bought the screen calipers some time ago, and they are excellent. I'd recommend purchasing because not being able to rotate them through 360 degrees can be quite restrictive, but they work just fine otherwise.

I've tried the other tools and they are quite useful but I haven't bought them.

Steve

I've had Screen Calipers for over a year. Very useful tool for many programs I use. The ability to enter custom units is the feature I use a lot.

Ed

Thanks for the link Brian! It was no more than one hour ago I was actually holding a pair of real calipers up to my screen while resizing something to true size in Photoshop. at least now I don't run the danger of damaging my screen :)

jonah