ErrorZilla – Useful error pages for Firefox is an extension that:
Changes the default error page from basically having a reload button to having the following choices: a google cache, an archival snapshot from the wayback machine, a ping, a trace route, and a whois lookup.
It also provides a “Try Again” option.
When a URL returns a 404 or Server Not Found error, the extension kicks in and displays a non-obstrusive popup providing the options described above. This should be a standard function for browsers. Highly recommended.
(via Google Operating System)