Free Ruler for Mac

Features:
Horizontal and vertical rulers, in any color you like.
Choose from pixel, millimeter, and inch units (press U to cycle).
Float rulers above other applications (press F to toggle).
Move windows independently or as a group (press G to toggle).
Show or hide the shadow behind rulers (press S to toggle).
Align rulers at mouse location (press O for origin).
Reset ruler locations with Command-R.