Advanced Google Search
When you use Google's advanced search, it's possible to limit your search to Creative Commons-licensed materials.  
Creative Commons Search
This search engine, will mainly locate Creative Commons-licensed materials.  It's not perfect since it will sometimes retrieve items that may not be covered by a CC license, and you should always verify the status of the work just to be sure. With this in mind, it is still a good tool for getting started.  The Creative Commons Search will search for web sites, images, videos, music tracks, and other online media that are covered by CC licenses.

MERLOT (Multimedia Educational Resource for Learning and Online Teaching). 
Some of the material found there is covered under CC licenses.  Be sure to double-check specific works for rights information.     
 

Car Photos

Need CC pictures of cars?   You can find such images here.

Creativity103.com

This site provides access to some very interesting CC-licensed background images. 

Getty Images

While Getty Images does not provide CC-licensed materials, it now offers the ability to use 35 million of its images for non-commercial purposes.  Simply go to their site by clicking on the link above, search for an image using key words, and you can limit your search to royalty-free options from the menu on top of the screen. 

OpenClipArt.org

In addition to locating images through the Creative Commons Search, Google Images, and Flickr, this site provides access to tens of thousands of images that are free to use under a Creative Commons Public Domain license.

PhotoEverywhere.co.uk 

This site provides access to Creative Commons-licensed images with a travel theme. 

Animal Photos

Use this site to find CC-licensed animal pictures.

Wikimedia Commons

Helpful resource which contains Creative Commons licensed and Public Domain images and other media.  One of the most helpful things about Wikimedia Commons is the Open-Source Ticket Request System (OTRS) which tracks the permissions to freely use media sources posted to the site. 

Moving Image Archive (Archive.org)
http://archive.org/details/movies

This site allows you to either stream or download film materials from a variety of sources.  Movies you'll find on this site are either covered under Creative Commons licenses or are in the public domain.  Please double check to be sure. 

Prelinger Archives: Free Movies

This collection contains downloadable films that are covered under a Creative Commons 0 license (Public Domain).  You can find, commercials, educational films, industrial films, some stock footage, and amateur films here. 

 

Audionautix

An individual artist, Jason Shaw, has released tracks on this site under a CC 3.0 license (attribution only). 

ccMixter.com

Tracks contained within this site are covered by CC licenses. Search function may not be the greatest, but they do have a large library of music to browse.

Community Audio

This is one of several music/audio projects hosted by the Internet Archive. Material can be downloaded under different Creative Commons licenses. Please be sure to double check what the license allows.

Free Music Archive (FMA)

The Free Music Archive allows legal downloads of Creative Commons-licensed music. The lists of music are curated by independent labels, terrestrial, and Internet radio stations.

Free Sound Project

Need some sound effects? You may be able to download and use some from this site. Please be sure to carefully read the CC license accompanying each file to see what uses are permitted.

Internet Archive Live Music Archive

A fairly long list of artists and bands are making live recordings of their music available for download on this site. The tracks are covered by a "trade friendly" which essentially means that they may only be used for non-commercial purposes. Please be sure to check the "Trade Friendly" page by EtreeWiki for further information.

Jamendo

This site contains both "royalty-free" as well as Creative Commons licensed music.  Contains over 400,000 tracks and is international in scope. 

Opsound

This site allows anyone to contribute sound files under a Creative Commons Attribution/Share Alike license.

Mutopia
In addition to providing access to pre-19th century music, you can also find some 20th and 21st century compositions here that are covered under CC licenses.

IMSLP
Contains some CC-licensed scores in addition to those in the public domain. 

Disclaimer: This tutorial on using public domain materials, Creative Commons licensed materials, and copyright law is provided for informational purposes only!  I am not a lawyer and cannot provide legal advice.  None of what you read in this tutorial should be construed as legal advice.  Should you require legal advice, please contact an attorney.