How can I see how a website has changed over time, or revisit "dead" websites?

Websites, by definition, are good, useful and accurate when they are up-to-date.  But sometimes it is significant to note change over time: what was different about a site at this time last year, or five, or twenty years ago?  Can we note the influences of a different political climate or different management by comparing a site's content, look and feel over time? 

If you've ever wanted to analyze a site as it has changed over time, or wanted to re-visit a site that no longer exists, there are tools that can make this task possible. 

• TimeTravel (searches the archives below and many others for prior versions of the website. They are called "mementos")

• Wayback Machine (Internet Archive)

• Library of Congress Web Archiving