The Wayback Machine
Now this is really cool. The Wayback Machine, the invention of the Internet Archive allows you to see any webpage archived from 1996 to just a few months ago.
This is useful if you’re looking for old information or if you wanted to see what the first iteration of a site that you designed and what it looked like.
About the Wayback Machine:
Browse through 85 billion web pages archived from 1996 to a few months ago. To start surfing the Wayback, type in the web address of a site or page where you would like to start, and press enter. Then select from the archived dates available. The resulting pages point to other archived pages at as close a date as possible. Keyword searching is not currently supported.
You can check it out at http://www.archive.org/web/web.php.