http://www.picpick.org/en/
PicPick is an all-in-one program that provides a full-featured screen capture tool, an intuitive image editor, a color picker, a color palette, a pixel- ruler, a protractor, a crosshair and even a whiteboard.
This is a very handy little tool, it hides in the system tray and you can assign your own keyboard shortcuts to launch the different tools.