Creative Commons (http://creativecommons.org) licences provide a flexible range of protections and freedoms for artists, authors and educators. Creative Commons is an alternative to copyright and is sometimes referred to as "copyleft".
The main Creative Commons website contains a number categories to assist finding content as well as a search facility.
The search facility provides by Creative Commons allows you to filter by "works that can be used commercially" and "works that can be modified, adapted or built upon" and passing the search query on to Google, Yahoo, Flickr and Blip.tv.
A discussion forum topic about Creative Commons is available here.
A short video clip on Creative Commons is provided below: