Prelinger Archive
This is found on the Internet Archive and offers access to many public domain movies and videos.
Audio
Digital History
A research project by the University of Texas, University of Houston, and University of Hawaii to find, list, and provide access to historical music that is in the public domain.
Freesound
Freesound is a collaborative database of audio samples released under Creative Commons licenses.
Partners in Rhyme
Partners in Rhyme is a collection of royalty-free music and sound effects.