Freesound

Freesound is a collaborative library of sound effects contributed by users, with a very large catalogue covering everything from field recordings to synthesized effects.

Its breadth in unusual sounds is what distinguishes it from commercial libraries: recordings of specific machines, environments and incidental noises that no commercial library would produce on spec.

Licences vary by upload — some require attribution, some are public domain — so checking the licence on each file individually is necessary rather than optional.