The wayback machine is a server which stores previous versions of millions of websites, effectively allowing you to search back in time.
One researcher used this tool to see how a government body had responded to some big news, it turned out that they had tried to cover their involvement up by changing their website. The stored versions of their site clearly showed their involvement, and how they had tried to lie on-line, as it retained the original incriminating statements.
Click on the image below to see a list of the stored versions of BBC.co.uk (sometimes it can be a bit slow):
Just to show how far bbc.co.uk has come check out a few of the welcome screens from almost 10 years ago:
"The Internet Archive is building a digital library of Internet sites and other cultural artifacts in digital form. Like a paper library, we provide free access to researchers, historians, scholars, and the general public."