SnapMeasure for Mac

From Nineblock Software:

SnapMeasure is a plug-in tool for Adobe Illustrator which addresses several weaknesses of the built-in measuring tool and will tremendously improve your productivity. It clearly snaps with visual feedback to elements on the page (no more measuring three and four times to be sure you are getting the right value). It can put the measurement values right where your eyes are -- next to the cursor. No more having to look up at the Info palette. It lets you scroll or zoom during a measure (no need to make sure the start and end points are both on the screen before you start measuring).

View larger image