Wikipedia
OpenGameArt.org
Open Game Art is a media repository intended for use with free software game projects.
Its purpose is to allow developers to easily replace programmer art with high-quality, appropriately licensed artwork. It accepts both 2D and 3D art, as well as sound effects and music, unlike similar projects such as ccMixter, which only deals with audio samples and songs, and The Freesound Project, which limits itself solely to samples.