Take into account that while it may be free as in “gratis”, its use may be restricted by its license.
For example, a CC-by is free of use, but if you use this artwork you need to explicitly specify the name of the author within the project.
Free Graphics for Video Games
Some resources for free graphics for video games.
OpenGameArt.org
Universal LPC Spritesheet Generator
It is licensed under CC-BY-SA 3.0.
Free Sound for Video Games
Some resources for free sounds for video games.
JSFXR
Free Music for Video Games
Some resources for free music for video games.