I take a lot of screen shots of the various device breakpoints for responsive design and it takes a while to prepare them. This tools allows me to get what I need quickly, and hopefully it can be helpful for showing your more visual clients what you mean by responsive design when a suite of products isn't at the ready.
ScrnShots is the best way to share your screenshots of web and screen based design. Upload as many screenshots as you want, embed them in your blog, discuss them with your contacts and become a better designer!