When you meet 404 not found, you may find web cache with this extension.
When you see the 404 not found pages while you are searching something, you can directory go to the backup page with "Wayback Machine extension". You may find web cache with this extension from The Internet Archive, Google Cache, Yahoo Cache, MSN cache(Bing Cache), CoralCDN, Gigablast, WebCite.
You can go to cache with right-click menu.
Update: Fixed a bug for new Wayback Machine beta interface.