The Wayback Machine is an initiative of the Internet Archive, a 501(c)(3) non-profit, building a digital library of Internet sites and other cultural artifacts in digital form. Other projects include Open Library and archive-it.org.
Wayback Machine Availability API – Build your own tools.
WordPress Broken Link Checker – Banish broken links.
404 Handler for Webmasters – Help users get where they were going.
Example: vision statement (2002)