Okta - Enterprise-grade identity management for all your apps, users & devices
Keycloak - Open Source Identity and Access Management for modern Applications and Services.
Auth0 - Auth0 is a program for people to get authentication and authorization services for their own business use.
Authelia - Authelia is an open-source authentication server providing SSO and two-factor for your applications running behind NGINX.
OneLogin - On-demand SSO, directory integration, user provisioning and more
Atlassian Crowd - Crowd is a single sign-on and user identity solution.