Freesound is a collaborative, community-uploaded database of over 730,000 free sound samples spanning nearly every sound category, maintained by Universitat Pompeu Fabra's Music Technology Group.
Value is sheer breadth and strong tag/search tooling for finding an obscure sound quickly across a huge pool of contributors.
Best for broad, exploratory searches across many sound types, when you're prepared to check each result's individual license.
Licensing varies per upload across three types (CC0 (no restrictions), CC-BY (commercial use allowed but attribution required), and CC-BY-NC (noncommercial only, so unsafe for monetized YouTube)), verify each downloaded sound's specific license before use.
Collaborative database of over 700k creative commons sound samples.
