The Internet Archive is a digital library containing books, audio, video, and other digital resources–including both information that originated on the Internet as well as older information that has since been digitalized. It partners with other organizations to archive their content, and also allows uploads from the community.
It is perhaps best known for the “Wayback Machine”, which preserves and allows you to view what many web pages looked like in the past. This obviously depends on the Archive having saved a copy of the page at that particular point, but for relatively popular sites which are no longer online, have switched owners or changed significantly, you’re likely to find an old version of the site.
Books published prior to 1923 (which are out of copyright in the US and therefore in the public domain) are available for anyone to download; newer books can be borrowed via the Archive’s Open Library.
The Archive currently holds at least 45 petabytes of information, and is scanning over 1000 books every day. It’s even archiving archiving this site, thanks to an integration set up by Micro.blog.