Apple Itunes : https://developer.apple.com/library/archive/documentation/Au...
last fm : https://www.last.fm/api
acoustid : https://acoustid.org/webservice
spotify : https://developer.spotify.com/documentation/web-api/
discogs : https://www.discogs.com/developers
deezer : https://developers.deezer.com/login?redirect=/api
music brainz : https://musicbrainz.org/doc/MusicBrainz_API
gracenote : https://developer.tmsapi.com/Sample_Code
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Additionally, why not list API rate limits?
Or some lookups don't need any security, like querying Apple Itunes does not require anything except for being nice and rate limiting.