SnapRuler

I typically use xScope for all of my on-screen pixel measuring needs, but recently, I've often found myseful using SnapRuler instead. It's very simple, but has three really useful features.

First, hitting shift immediately snaps the measuring rectangle to the nearest visible edge (like the border of a button or window). Second, once you've measured something, you can copy the values as CSS or Objective-C code, and directly paste them into your text file. And third, you can easily resize your selection rectangle by single pixels using the arrow keys (something I have to do often when I'm working with a trackpad).

Both xScope and SnapRuler are really nifty, and get along nicely with each other.