Keycloak is described as 'Open Source Identity and Access Management for modern Applications and Services' and is a Identity Management Tool in the category. There are more than 25 alternatives to Keycloak for a variety of platforms, including Online / Web-based, Self-Hosted solutions, Windows, SaaS and Linux. The best alternative is FreeIPA, which is both free and Open Source. Other great apps like Keycloak are FusionAuth (Paid), Auth0 (Freemium), Okta (Freemium) and CAS (Free, Open Source).
LemonlLDAP::NG is a modular Web-SSO based on Apache::Session modules. It simplifies the build of a protected area with a few changes in the application. It manages both authentication and authorization and provides headers for accounting.
Amazon Cognito lets you add user sign-up/sign-in and access control to your web and mobile apps quickly and easily. Cognito scales to millions of users, and supports sign-in with social identity providers such as FB, Google, and Amazon, and SAML 2.0.
OpenID Connect / OAuth 2 server for single sign-on, identity provision and federation, and securing web APIs with access tokens. Supports profiles for fintech applications, eHealth, eGovernment and others.