Recommendation for a 2D screen ruler

I am developing on a large monitor and would like to see, at a glance, how different parts of my application look at different screen resolutions.

I'm not interested in a utility which resizes my application or windows inside my application; I'm more interested in some sort of 2D screen overlay that can be set to different dimensions, and which I can move around the screen.

I have seen plenty of screen rulers, but I'd rather not have the hassle of measuring horizontally, then flipping the ruler, then measuring vertically. I'd just like a transparent, floating "box" that I can move around the screen so that I can tell, for example, if my dialogs are getting too tall.

I use this one (Opera widget)

(Warning: not necessarily the best solution.)

My quick-and-dirty way to do this was to create a desktop background with common monitor resolutions on it in different colours.

It's worked well enough. I think if a dialog box is getting really large with a ton of controls, maybe filling an 800x600 screen, it is most likely a candidate for GUI refactoring.

Check out xScope from IconFactory. http://xscopeapp.com I use it all the time for development layouts. It's got several tools, including rulers, guides... and a "Frames" tool that is perfect for previewing other screen resolutions like you want to do.