Need a webpage from 1992? The Grateful Dead's 1966 Concert at the Fillmore in San Francisco? A book published in 1762 outlining all the laws passed in the 1400s in England? The Internet Archive is where you need to be!
The Internet Archive includes the Wayback Machine, the history of over 351 billion web pages on the Internet. It searches Google Books for public content. It contains political ads, a TV News archive from 2009 to today, old time radio shows... and so much more!
The Hathi Trust is a collaborative project of research universities and other institutions to digitize their collections. There are many full-text resources available without a login. If you are looking for something a little obscure, this is a great place to start!