Creative Commons/Public Domain

The Creative Commons Foundation has its own search engine and an overview of the different kinds of licenses. This page lists more specialized collections.

Images (stock, artistic)
Royalty-free stock photos from Getty Images

Archival images
British Library on Flickr
Wellcome Trust: exquisite collections in the history of science and medicine, mostly CC licensed

Archival video footage
Prelinger Archives, a huge collection of ads, educational and industrial films from the 1920s-1970s

Free Music Archive, operated by WFMU in Seattle. Make sure you check the license for each song.
Use the CC search engine to find shareable tracks on SoundCloud

Other audio
Sounds from the British Library