GrabzIt has just launched their screenshot service, which allows developers to easily take screenshots of websites for free using the open source GrabzIt PHP Library. To take a screenshot with GrabzIt you first need to register to get your application key and secret, once done you will be up and running in no time.

Their (quick) example shows you how to configure the library object with your keys and call the "take picture" method to grab the shot of a given site URL. You get back an ID that can then be used to fetch the screenshot from the service as needed (pulled down and put into an image file with a file_put_contents.