Online Multimedia Collections 

Prison Door, 1910, Angel Island (Calif.)
Source: University of California, San Diego
Retrieved from Artstor

These are some important words with which you should familiarize yourself before searching for usable content on the web:

Attribution - If a creator asks for attribution, you must reference them to let the audience know where you got the material.

Commercial Uses - If you create a video for a class project, you are not creating something for commercial use.  If, however, you sell your project to a corporation for them to use in an ad campaign, that would be a commercial use.  Many creators won't allow this.

Free - Just because an image, audio clip or sound byte is "free" might mean it's free to view or listen to - not necessarily free to use, download, or reproduce.

Derivatives - If a site says you may not produce derivatives, that means you may not alter the work in any way.  You may use smaller clips or show details of the work, but you may not, for instance, Photoshop a picture or add another instrumental part to a musical score.

Fair use - Just like it sounds: using the material that you've found fairly within copyright restrictions.  Often, if an item is used for the public good (in educational institutions, or to increase public awareness about an important issue), and it is non-profit and not excessively distributed, the use is considered fair. 

Open Access - If something is labeled "Open Access," it's apart of a movement that allows users to freely adopt, adapt, and distribute content.

Public Domain - If a site claims to be in the public domain, that means their materials are no longer under copyright (or, rather, whose rights are owned by the public).  Many older materials are in the public domain because their copyrights have expired, and anything published before 1923 will be in the public domain.  These sites are free for use and reproduction.