One of my favorite sites for a while now is archive.org (about). They focus on hosting public domain or creative commons media. A certain percentage of the archives is old material that has seen its copyright expire, while the other part of the collection was intended for public use from its conception (stock video, home videos). Among other things, this means that you can use this material in your own works without fear of being sued for copyright infringement. This is one of the reasons why classical and baroque music is used so much in television, movies and advertising: the composers are long dead and gone so there is no hassle with contacting the owner of the copyright, paying royalties, paying lawyer fees, etc.

Examples:

• If you click this, you can download (or just stream) 298 individual episodes of the old-time radio detective series Dragnet (1949-1955)
• This is a hilarious video (everything from 1990 is hilarious) about the release of Windows 3.0 and how it related to competing products at the time.
• 1949: Care of the Skin by Encyclopaedia Britannica Films. This ten-minute video covers the basics for teen cleanliness such as "the proper way to wash" and "the correct use of soap."
• Thomas Edison's oldest known voice recording is on this page, a recording of the inventor himself from 1888.
• Here is an opensource .pdf file of all the Beatles songs in sheet music format. 292 pages worth, free for download.
• On this page, you can watch someone beat Super Metroid for the SNES in 1:05 while collecting 100% of all items. I'm not sure why you would do that, but you could. You could even download high quality DivX versions of the videos and burn them to DVD.
• Searching my birthday, I found "Secret White House Recordings" of Lyndon B. Johnson from May 4, 1966

All of the material is downloadable, and all of it contains copyright information right there on the page. Some of it is free for use (no rights reserved), some of it is posted under "some rights reserved." Theoretically, all of our current entertainment will someday lose its copyright (although the lawmakers keep extending it, as much as 100 years now). One day, someone could use clips from E.T. in a film and not have to pay royalties.

(Another reason I like archive.org is they host a bunch of my music, but I'll get to that later)