I think its cool "ErrorZilla
is a Firefox extension that replaces the standard "server not found" page and similar page errors with something more useful. ErrorZilla adds a reload button, Google Cache, Web archive, and three buttons for: ping, trace and whois. The most useful is the "Google Cache" button that lets you view the page even if the server is down, assuming that the page has been indexed by Google."
copy paste from here http://googlesystem.blogspot.com/2006/09/errorzilla-better-http-error-page.html