These sources contain--or link to sources of--a range of media.
Hathi Trust - A digital library dedicated to the preservation of the culture record developed in partnership with research institutions.
Internet Archive - A digital library of text, music, video and more.
Guide to Finding Interesting Public Domain Works Online from Public Domain Review. Includes many more sources for public domain and Creative Commons media. Also includes a good description of Creative Commons and public domain works.
Wikimedia Commons A database of images, video and sound.