Go here to find works that are licensed through Creative Commons, which means that you can use the work for free as long as you follow the guidelines of the license. There are six different licenses, which you can read more about here: http://creativecommons.org/licenses/
You can search for CC licensed work on this search engine. Just choose whether you will modify, adapt or build upon the work, and if you will use the work for commercial purposes. Choose what you want to find. For example type “computers” in the search box if you want videos, pictures or audio that deal with computers. Click on the website through which you want to find the content. A list of CC licensed work will appear.