The Wayback Machine (http://www.archive.org/), also known as the Internet Archive, is always a popular site on my search workshops. It is a fantastic way of discovering how web pages looked in the past and for tracking down documents that are no longer on the live web.
It isn’t 100% guaranteed to have what you are looking for and at present you need the URL of the web site or document in order to use it. People often ask if keyword searching is possible; it isn’t at the moment but it will be.
The Internet Archive has received support from the Laura and John Arnold Foundation (LJAF) and will be re-building the Wayback Machine. When it is completed in 2017, the next generation Wayback Machine will have more webpages that are easier to find and will include keyword indexing of homepages.
Further details of the rebuild are on the Internet Archive blog at http://blog.archive.org/2015/10/21/grant-to-develop-the-next-generation-wayback-machine/