I looked into this recently too and found there are a few solid alternatives besides IdentityServer. Keycloak is a strong open-source option from Red Hat that supports SSO, LDAP, and social logins. OpenIddict is another good one if you're staying within the ASP.NET Core ecosystem. For more enterprise-heavy needs, WSO2 Identity Server and ForgeRock are worth a look, though they come with a learning curve. IdentityServer might dominate search results, but it’s not the only game in town.