Plataformatec/devise. EppO/rolify. Ryanb/cancan. OmniAuth: Ruby Authentication Framework. What is Omniauth?
The web application landscape has changed drastically in the past few years. Most users login in to dozens, sometimes hundreds of services each day; sites are no longer silos unto themselves and cannot reasonably expect users to create a unique login and password for each service. This is where OmniAuth comes in. OmniAuth is a library that standardizes multi-provider authentication for web applications.