EuropeanaOffers over 20 million digital images from Europe's museums, libraries, archives and audio-visual collections -- all of which are in the public domain.
AUDIO
Here are some sites where you can download sound & music files.
an interactive library of high-quality, legal audio downloads. Conditions of use vary and are determined by the rightsholders themselves (see their FAQ for details).
ccMixter is a community music site featuring remixes licensed under Creative Commons where you can listen to, sample, mash-up, or interact with music in whatever way you want.
All the music on jamendo is available under one of the six Creative Commons licenses. They authorize free download and enable the artists to promote their music while protecting their rights.
VIDEO
Internet Archive: Moving Image ArchiveThis library contains thousands of digital movies uploaded by Archive users which range from classic full-length films, to daily alternative news broadcasts, to cartoons and concerts. Many of these videos are available for free download.
YouTubeAfter your initial search, click on the link to filter your results, then click "creative commons" under Features.