The internet wayback machine is the website [web.archive.org] which you can refer to get copies of websites in different period of times if it is available. For instance, you can get see the main page of Yahoo! from 2002 onwards.
For me it is very interesting because it brings me back to old days and the sweet memory of very early era of the internet. When I opened Yahoo! main page in the period of 2002 to 2005, it truly backs me to those days that I had a P3 PC with 56K modem.
Apart from the nostalgy part, it is also a good idea to keep various copies of websites for webmasters. However, sadly, there is no automatic mechnisem to submit the pages to web.archive.org as far as I am aware and the only provided API can be used to check the availability of a website copy in certain period of time.
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