There are many ways to find Creative Commons licensed work! A good starting point is the Creative Commons web site. They offer a search interface that gives you links to nearly a dozen search tools to find images, clipart, video, music, and other media.

• Wikimeda Commons is a great place to search for clearly licensed media and images. Note that some resources are licensed with a GNU license, which may be difficult to adapt to your purposes.
• Flickr Advanced Search allows you to limit to CC content only
• Google Advanced Search also allows you to filter search results roughly by re-use licensing. Use the "usage rights" filter at the bottom.
• Getty Images offers 'open content' that is available without charge for any purpose. Click here to search the collection. 
• Free Music Archive
• The National Library of Medicine has a series of beautiful historical anatomical images online in the public domain.
• Collins Library's page on finding Copyright Free images is a good place to start an image search.
• The Public Domain Review maintains collections of films, audio, images and texts which are no longer protected by copyright.