Measuring elements on the screen quickly and easily can be really important for all sorts of tasks. I often use the excellent Free Ruler app for this but don't always want to waste time opening it up.
For super-quick measurements just use the built-in screen capture command (Cmd-Shift-4) to drag and measure the element you're interested in instead. Click and drag to measure and to avoid taking a screenshot simply press the Escape key before releasing the mouse button.
I'd not really thought of this as a hint until a fellow designer was astounded when then they saw me doing this, so I thought I would share!
[crarko adds: Free Ruler has been mentioned before but this is a nice quick alternative.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20100719093135976