NB: Royalty free is not the same as free or Public Domain.
Royalty free media have to be paid for unless otherwise stated.
Wikimedia Commons – most material seems to end up here eventually, no matter what type 🙂
Flickr Commons – material from ordinary photographers that’s been released into the wild. Please check which licence should be applied.
Geograph – a project trying to photograph every square kilometre of the British Isles. All images have Creative Commons licences.
Metropolitan Museum of Art – 400,000 images released into the public domain. Look for OASC (Open Access for Scholarly Content).
Wikimedia Commons – images collated from other sites, so it’s a good collection. Please check which licence should be applied.
British Pathe – films are accessible through YouTube. Would need to be unblocked.
Sounds: music, sound effects
BigSoundBank – real world recordings
Freesound blog – sound effects available through Creative Commons
PDSounds – all in the Public Domain
FreeSFX – excellent collection of sound effects. Licensing terms clearly state that sounds can be used for any purpose, so long as connection to FreeSFX is included.