Devise
参考：
plataformatec/devise: Flexible authentication solution for Rails with Warden. – GitHub
DeviseInvitable
Devise にメール招待機能を追加する
参考：
scambra/devise_invitable: An invitation strategy for devise – GitHub
OmniAuth
参考：
omniauth/omniauth: OmniAuth is a flexible authentication system utilizing Rack middleware – GitHub
初心者が Rails + Devise で Todo アプリ作る Omniauth編 | freedom-man
Sorcery
参考：
Sorcery/sorcery: Magical Authentication – GitHub
Pundit
参考：
varvet/pundit: Minimal authorization through OO design and pure Ruby classes – GitHub
Pundit + Rails で認可の仕組みをシンプルに作る – Qiita
権限管理の gem、Pundit の紹介 | Rubyist Magazine (るびま)
Using Pundit for authorization in Rails | Tales from a Codemancer
Authlogic
参考：
binarylogic/authlogic: A simple ruby authentication solution – GitHub
Authlogic + Rails で認証の仕組みをシンプルに作る – Qiita
Rodauth
参考：
jeremyevans/rodauth: Authentication and Account Management Framework for Rack Applications – GitHub
認証 gem ‘Rodauth’ README 翻訳 | TechRacho
CanCanCan
参考：
CanCanCommunity/cancancan: The authorization Gem for Ruby on Rails – GitHub