I love it when I can guess how to do something new in a program. Skim just did this for me - I needed to figure out how big the rectangle of text in a paper I'm working on is, in inches. I looked for rulers, and then decided I'd just see if I get any size feedback when I make a rectangular selection. Turns out there's a nice feedback status line in the bottom right - clicking toggles between measurement in pts and in inches.
I like how this solves my problem without adding a lot of new UI for measurement - no rulers, no extra tool to measure length.