The largest source of media on the internet is probably the file sharing that goes on using BitTorrent. BitTorrent is a protocol for how to share files using a peer to peer computing network. That means everyone who is downloading the file is also a source for the file. The default setting is set to share the file So if you download via bitTorrent, you will most likely also be sharing the file with others.

If you would like a good bittorent client for downloading via BitTorrent. You can get free,
open source bittorrent software called uTorrent at

http://www.utorrent.com/

You can use this software to download files that have been set up
for file sharing via bittorrent.

A long list of other bittorrent applications is available at

http://www.answers.com/topic/list-of-bittorrent-software

Many of the files you will find on these search engines
are pirated and not legal for downloading in most countries.
You should be aware that the recording industry will sometimes
set up dummy files and track people who are sharing their files.
This can lead to legal action.

SO DON’T STEAL MEDIA PROTECTED BY COPYRIGHT!!!

You should also be able to find media that is protected by
a Creative Commons license which is legal for sharing, reuse
and remixing if the content author so designates. These are often
marked with a CC instead of “All Rights Restricted”.

These kind of files can be found at sites like Legaltorrents.com.