These sites provide copyright friendly content.  While some resources come without any inherent copyright claim, others have some restrictions on how they may be used.  Be sure to read the terms of service on each site to ensure the resources are being used appropriately.  Please note, some of these sites also provide paid content as well.

Images and Graphics
Open Clip Art Library http://openclipart.org/
School Clip Art http://www.school-clip-art.com/
Pics4Learning http://pics4learning.com/
Pixabay http://pixabay.com/
Flickr – Creative Commons http://www.flickr.com/creativecommons/
Everystockphoto.com http://www.everystockphoto.com/
WP Clipart http://www.wpclipart.com
Unrestricted Stock http://unrestrictedstock.com/
Free Digital Photos http://www.freedigitalphotos.net/
Clker.com http://www.clker.com/
Stock Photos for Free http://www.stockphotosforfree.com/
Photos For Class http://www.photosforclass.com/
Pexels https://www.pexels.com/
Audio and Sound F/X
Freesound http://www.freesound.org/
Free Stock Music http://www.freestockmusic.com/
Freeplay Music http://www.freeplaymusic.com
AudionautiX http://audionautix.com/
YouTube Audio Library http://www.youtube.com/audiolibrary
Incompetech http://incompetech.com/music/royalty-free/
PacDV http://www.pacdv.com/sounds/free-music.html
Video
Footage Crate http://footagecrate.com/
MotionElements http://www.motionelements.com/free/stock-footage
VidsPlay http://www.vidsplay.com/
Motion Backgrounds http://www.motionbackgroundsforfree.com/
Stock Footage for Free http://www.stockfootageforfree.com/
Movietools http://movietools.info/