Auth0 - Auth0 is a program for people to get authentication and authorization services for their own business use.
Okta - Enterprise-grade identity management for all your apps, users & devices
Firebase Authentication - Application and Data, Application Utilities, and User Management and Authentication
Duo Security - Duo Security provides cloud-based two-factor authentication. Duo’s technology can be deployed to protect users, data, and applications from breaches, credential theft, and account takeover.
OmniAuth - OmniAuth is a flexible authentication system utilizing Rack middleware.
OneLogin - On-demand SSO, directory integration, user provisioning and more