From: http://www.techandlife.com/tag/windows-snipping-tool/

You’ll occasionally need to be able to draw text on the image, highlight parts of the image, 
draw arrows to point out certain features, and obfuscate (blur) any sensitive text

From: http://www.picpick.org/en/

Take screenshots of an entire screen, an active window, the scrolling windows and any specific 
region of your desktop, etc.
Annotate and highlight your images:
text, arrows, shapes and more
Variety of graphic design accessories including color picker, color palette, pixel ruler, protractor, 
crosshair, magnifier, whiteboard