Orchard Core is an open-source modular and multi-tenant application framework built with ASP.NET Core, and a content management system (CMS) built on top of that framework.
Orchard is a free, open source, community-focused Content Management System built on the ASP.NET MVC platform. Check out the next generation of this software built on ASP.NET Core: https://github.com/OrchardCMS/OrchardCore
Silverstripe CMS - this is a module for Silverstripe Framework rather than a standalone app. Use https://github.com/silverstripe/silverstripe-installer/ to set this up.