The Internet Archive:
Have you ever clicked on a web link only to get the dreaded “404 Document not found” (dead page) message? Have you wanted to see what that page looked like when it was alive? Well, now you’re in luck.
Recently the Internet Archive and Mozilla announced “No More 404s”, an experiment to help you to see archived versions of dead web pages in your Firefox browser. Using the “No More 404s” Firefox add-on you are given the option to retrieve archived versions of web pages from the Internet Archive’s 20-year store of more than 490 billion web captures available via the Wayback Machine.