Amazon Cognito - Amazon Cognito lets you add user sign-up, sign-in, and access control to your web and mobile apps quickly and easily. It scales to millions of users and supports sign-in with social identity providers and enterprise identity providers via SAML 2.0.
Okta - Enterprise-grade identity management for all your apps, users & devices
Devise - Flexible authentication solution for Rails with Warden.
Auth0 - Auth0 is a program for people to get authentication and authorization services for their own business use.
Atlassian Crowd - Crowd is a single sign-on and user identity solution.
OneLogin - On-demand SSO, directory integration, user provisioning and more