There are a few sites I use all of the time, and sometimes I forget to share these gems with other teachers. The Internet Archive is dedicated to collecting Web sites to allow permanent access to these sites for researchers.
One of their easy-to-use tools is the Wayback Machine. Simply type the URL of a site that no longer works, and you will be presented with a list of clickable dates that the page was cached and saved to the Internet Archive. Sometimes the images and external links are broken, but it just wonderful to be able to get at information you thought was gone forever!
The Internet Archive and the Wayback Machine may be found here: http://archive.org/