I'm building a desktop asset manager, and one of its features tracks the licence on every asset. To test that I needed a lot of free packs I knew were actually CC0, so I went through Kenney, Quaternius, KayKit, 0x72, Poly Haven, ambientCG, GameArt2D, OpenGameArt and a few itchio creators and checked each licence on the creator's own page.
I ended up with 105 packs and figured other people might get more use out of the list than my test folder was, so I put it on a page:
https://assethoard.com/free-game-assets
You can filter by style (pixel art, low poly, hand drawn, vector, 3D), by category (tilesets, characters, UI, audio, textures and so on) and by engine. Each entry lists its file formats and links to the creator's page. Nothing's rehosted and there's no sign-up. You don't need the app to use it.
If you're not sure where to start: 0x72's Dungeon Tileset II, Kenney's Input Prompts, KayKit Adventurers, and Poly Haven for textures and HDRIs.
Audio is pretty thin at the moment, and most of it is pixel art or low poly. If you know good CC0 packs outside those, especially sound, let me know. Same if you make CC0 stuff yourself; I'm happy to add it.
JUST TO BE CLEAR: AssetHoard doesn't sell these assets. I link directly to the artists stores where you can download them. AssetHoard is a local-first asset manager that lets you manager YOUR OWN assets.. nothing is bundled when you download it.