Markman

Markman－－A efficient and sweet mark, measuring tool for designers,support both Windows&Mac OSX.
you can mark and measure like Lengths,Coordinates and Rectangles,Colors,Texts...
It also has some sweet features:
Length Auto Detection
Drag to remove
Support Main Stream Formats: Support PSD（with Max Compatibility）、PNG、BMP、JPG
Auto Reloading: If the image file is modified, MarkMan will reload the image file automatically. You can design and spec side by side.
Style Customizable: Control Color, text size, length unit, etc. with context menu.
Support @2x Retina image: Zoom out image 50% when the image name is tailed with '@2x', so the measured length would be doubled.

DOWNLOAD: http://www.getmarkman.com/