Free Game Resources
Graphics
Clker.com - royalty free public domain clip art
OpenClipArt - public domain and otherwise DFSG-free clip art
Image *After - GPLed images and textures
BurningWell - Public Domain Images/Textures
Vecteezy - Free Vector Art
BlendSwap - Free (Blender) models
Ryzom - 13GB of CC-BY-SA 3.0 stuff
Fonts
OpenFontLibrary - DFSG-free fonts (mostly OFL or public domain)
Sounds
Free Sound Samples from the OLPC project (CC-BY 3.0 license)
PDSounds Public Domain Sounds
Sound Bible Mixed free and non-free sounds but license really visible for each sample
OpenOffice.org gallery: /usr/lib/openoffice/share/gallery/sounds
Music
Jamendo - music licensed through one of several Creative Commons licenses or the Free Art License (filter by free licenses). Sometimes there's a problem with downloading as Ogg Vorbis, but there's a work-around with a direct URL.
Wikipedia list of full length copyleft/public domain musical works available on Wikipedia or Wikimedia Commons
The Mutopia Project: free music scores and midis (CC-BY and CC-BY-SA)
Internet Archive's Open Source Audio: various recordings, but apparently most files lack a license statement
Miscellaneous
Wiki on FreeGameDev - Free 3D and 2D art and audio resources
Free Game Resources in Löve Club
- Other games in Debian main!