Duende IdentityServer is an open source .NET implementation of the OpenID Connect specification and acts as an Identity Provider (OpenID Connect Provider). IdentityServer is designed as an OWIN/Katana component and can be run embedded or as a standalone project. IdentityServer allows implementation of single sign-on and access control for modern web applications and APIs using protocols such as OpenID Connect and OAuth2. It supports a wide range of clients such as mobile, web, SPAs (Single Page Application) and desktop applications and is extensible to allow integration in new and existing architectures.
Unit4 Identity Services (U4IDS) is built on top of IdentityServer. For more information on the Duende IdentityServer see the IdentityServer documentation page on github.
The IdentityServer project is fronted by Dominick Baier and Brock Allen. See Dominick Baier's blog for more information on IdentityServer related topics.