What Freesound is
Freesound is a collaborative database of Creative Commons sounds, run since 2005 by the Music Technology Group of Universitat Pompeu Fabra in Barcelona. Hundreds of thousands of recordists have uploaded drum hits, synth notes, field recordings, foley and loops, and every upload is analysed for tempo, key and loudness.
Best for
Almost anything: drum one-shots, foley, field recordings, synth and instrument notes, vocal snippets and effects. Its tempo and key analysis makes it a good source for loops and tuned sounds.
Licence and what it means for a release
Each sound has its own licence chosen by the uploader: CC0, CC BY or CC BY-NC (in 3.0 or 4.0 versions), plus Sampling+ on some older uploads. CC0 and CC BY sounds can go into a commercial release (CC BY with a credit); CC BY-NC sounds can't.
How Sound Server searches it
Through Freesound's official API. Previews (MP3 and Ogg) stream to anyone; original WAV, AIFF or FLAC files need a free Freesound account, so results link to the sound's page. When the server has no Freesound key, Openverse still returns Freesound sounds.