The Internet Archive's Wayback Machine  allows you to search for Web pages no longer accessible to the public. Browse by date through over 150 billion pages archived since 1996.
The Archive's mission is to help preserve digital artifacts and create a publicly accessible Internet library for researchers, historians, and scholars. The Archive collaborates with institutions including the Library of Congress and the Smithsonian.
It can take 6 to 24 months for pages to appear in the Wayback Machine after they are collected. The Archive does not collect pages that require a password to access, pages tagged for "robot exclusion" by their owners, pages that are only accessible when a person types into and sends a form, or pages on secure servers. If a site owner requests removal of a Web site, that site will be excluded from the Wayback Machine.
Note: Another source of previously published Web pages is Google. The search engine maintains a "cache" — a version of the page from when they last indexed it. Access is via the "cached" link that shows up under a search result.