Apis.Guru is an interesting one, basically it’s a massive list of APIs that MUST have a specification document to be listed.
As of the date of this article there are two forerunners in the specification document game: Swagger and OpenAPI. They are not really competing types, more like versions. Swagger is a version 2.0 doc and OpenAPI is v3.0 at the moment.
Spec docs are incredibly useful, not only can humans read them in order to understand the intricacies of an API they may be interfacing with but more importantly computers can ingest/ consume them and spin up more precise machine-written code. Theoretically interfaces can be spun up on the fly although I don’t know if that’s being really done at the moment. Might be a future thing to watch for!