And because it's KM you can tweak it how you want -- change the selection keys, add more points, load the clipboard with some or all of the results... Even use it to auto-fill the X and Y fields in the "Move or Click a Mouse" action!
We should also mention KM's built-in "Mouse Display" palette (in the Editor's Window menu), which let's you set your "zero" to any of the four display corners or front window corners.
And with a bit of manoeuvring you can use it to measure other distances -- for example, put another window in front of your window of interest, position a corner over the object you want to measure from, put the cursor over the target. Don't forget the "Lock" button, which gives you 5 seconds to place the pointer then freezes the displayed coordinates.