Before I talked about DevToolbar, well that can be a bit much for the average user.
If you quickly want to measure something on your screen in whateva format, you can use this little nifty tool called JRuler.
Hang a shortcut in your Quick Launch bar and voila always ready.
JRuler
Download file (142KB)
I tell you what, it has more features and clever useful ways to use then you think.
Tnx to Brian again!
