Duende IdentityServer is a flexible and open-source identity and access management solution that enables you to customize workflows, UI, UX, business logic, and data. Developed by Duende Software, it gives you full control over your client applications, users, and APIs. It is a self-hosted OAuth server for .NET that provides dynamic authentication providers, resource isolation, and priority developer support. Duende IdentityServer can be integrated with any database, including SQL Server, and supports integration with ASP.NET Core Identity. It is worth noting that if you plan to use Duende IdentityServer for production purposes, you might need to pay a license fee to Duende Software.