Okta - Enterprise-grade identity management for all your apps, users & devices
Spring Security - The Spring portfolio has many projects, including Spring Framework, Spring IO Platform, Spring Cloud, Spring Boot, Spring Data, Spring Security...
OmniAuth - OmniAuth is a flexible authentication system utilizing Rack middleware.
Firebase Authentication - Application and Data, Application Utilities, and User Management and Authentication
oauth.io - OAuth that just works. Integrate 100+ OAuth providers in minutes.
JSON Web Token - JSON Web Tokens are an open, industry standard RFC 7519 method for representing claims securely between two parties.