OpenGameArt.org
A community library of sprites, tiles, models, music, and sound effects with author and license information on each entry.
Why use it
It helps prototypes and low-budget projects discover reusable content and study asset organization in open games.
Where it fits
Use art, animation, audio, levels, and narrative to make the game easy to read and its world convincing.
What to check
Licenses vary and may require attribution, share-alike, or prevent some combinations; retain source and license snapshots for every item.