Ask YC: Is there a utility that takes thumbnail style 'screenshots' of webpages?

I have noticed more and more different websites displaying little thumbnail screenshot images of entire webpages, for instance (picking something a total random:

http://dzone.com/links/gumbo_inline_twoway_data_binding.html

Notice the snapshot image of the website it links to in the center of the page. How do you do that? Its appearing in so many places, I am guessing there must be a library for it somewhere, but I have no idea what!