How to Implement API Key Authentication in Rails Without Devise

It's gotta be at least once a week that I see somebody on /r/rails or Stack Overflow asking how to implement API key authentication using Ruby on Rails. Most of the time they ask how to do it with the ubiquitous Devise gem, or the accepted answer points them in that direction. And everytime I think to myself, "why is everybody using Devise for something as simple as API key authentication?"Devise was created to handle browser-based authentication via cookies for run-of-the-mill, non-API, Rails applications. Devise really shines in its simple and secure session management, ready-to-go view and mailer templates, and support for things like SSO and OAuth using OmniAuth.