Want to view a page that no longer exists, or see how it has changed over time? Check out the new version of the Wayback Machine, provided by the Internet Archive. "It offers web researchers several new features and a clean, no-nonsense user interface. Wayback is an essential internet research tool and should be one of the first resources discussed when teaching basic web-based research skills. It offers more than 150 billion archived web pages with some material dating back to 1996."
Note: "There is a delay of at least 6 months before what has been archived becomes accessible, and it’s not keyword searchable. Nevertheless, Wayback is the easiest, quickest, and in some cases only chance a researcher has in trying to find material that is no longer available on the web."
You can access the beta at http://www.waybackmachine.org