Keycloak - Open Source Identity and Access Management for modern Applications and Services.
IdentityServer - An open-source, standards-compliant, and flexible OpenID Connect and OAuth 2.x framework for ASP.NET Core
DotNetOpenAuth - DotNetOpenAuth is a free-to-use compiled library that comes with the real support to your site visitor to login with the help of openIDs via getting control of the ASP.NET control onto the page.
Ory - Developer-first Access Management
Okta Workforce Identity - Okta Workforce Identity is utility software for enterprises that provides a new way of work that enable your employees, contractors, and partners to use technology on any device.
openiddict - OpenIddict aims at providing a versatile solution to implement an OpenID Connect server and token validation in any ASP.NET Core