What OpenGameArt is
OpenGameArt is a community library of free game assets, including sound effects, ambiences, chiptune and game music.
Best for
Game sound effects and retro or chiptune sounds.
Licence and what it means for a release
Each entry has its own licence: CC0, CC BY, CC BY-SA, GPL or OGA-BY (an attribution licence close to CC BY). CC0, CC BY and OGA-BY allow commercial use (with credit for the BY ones); SA and GPL bring share-alike conditions.
How Sound Server searches it
From OpenGameArt's search pages, read and cached; each listing links to the entry with its files.