IdentityServer by Duende Software: OpenID Connect and OAuth 2.x

used to be free and open source IdentityServer (for/by .NET)

now it is paid product/service, with support

IdentityServer by Duende Software

Community Edition by Duende Software

A free license that is feature equivalent to our Enterprise Edition

Duende Software
The most flexible and standards-compliant OpenID Connect and OAuth 2.0 framework for ASP.NET Core.

DuendeSoftware/IdentityServer: The most flexible and standards-compliant OpenID Connect and OAuth 2.x framework for ASP.NET Core @GitHub

pricing