A Creative Commons license is far less restrictive than a traditional copyright, and allows people to adapt, remix, and repurpose the original work. While there are several different variations of CC licenses, searching Creative Commons media can be a great way of finding images, video, and sounds that you can use in your own work -- provided you abide by the particular license and attribute the original author. The search engine CC Search allows you to search the following services:
- Google Images
- Open Clip Art Library
- Wikimedia Commons
Despite their recent addition of Creative Commons licenses, Vimeo is not (yet) included in the search engine, presumably because there's no way to search by CC-licensed content at Vimeo (this functionality is reportedly in the works). And it would be great if CC Search could somehow pull in media from all of the services at once, sorted by popularity (though this would not be an easy undertaking from a programmatic standpoint).
For more on creative commons, copyright, and remixing, see the excellent Danish documentary Good Copy Bad Copy (which, true to its topic, is watchable in its entirety, for free).
Finally, in the spirit of participatory culture, No Film School is in fact CC-licensed under the BY-NC-SA variant (the same goes for the DSLR eBook). So in addition to using CC Search to find content, I'd also encourage you to license your own work for others to adapt and build upon...