This article discusses the top five open source Single Sign-On (SSO) solutions and their pros, cons, and what to watch out for when choosing one. The article also highlights the importance of considering a hosted SSO provider instead of an open source solution in certain cases. Keycloak, Central Authentication Service (CAS), FreeIPA, Authelia, and IdentityServer are the five open source SSO providers covered in this article. Each provider has its unique features, benefits, and drawbacks that developers should consider when choosing an SSO solution for their applications. The article also emphasizes the need to look for protocol support, security features, specific IdP support, user-friendly login flows, regular updates, high availability, seamless integration, and cost-effectiveness while selecting an open source SSO solution.