Yesterday I mentioned Specctr for facilitating designer/developer hand-off. Reader Rick Johanson suggested SpecKing as an alternative:
Making the design specs in Photoshop can be a real time consuming task. SpecKing simplifies this process and lets you instantly measure any sizes, margins, and font properties in any PSD file. Your design specs can be ready in an instant, with just a few clicks.
[YouTube]
I have this guy installed and it works as developer is promising, here is my user side review http://goo.gl/8hAJEz if someone is interested.