ReadMe.io currently focuses more on the "front end" of the API. We can ingest in-line comments made with the apiDoc.js [1] standard, and then use that description to build an API explorer and reference docs automatically. We like apiDoc because of how concise the comments are, and because they are kept very close to the code (no extra files to maintain). It's not as common as some of the other standards mentioned here, but it's — by far — the fastest and most straightforward way to describe an API.

[1] http://apidocjs.com/

Edit: this is how it's implemented within ReadMe.io: http://readme-sync.readme.io/v1.0/docs