Mono branch of Microsoft's ASP.NET WebStack
C1 CMS Foundation - .NET based, open source and a bundle of joy!
DNN (formerly DotNetNuke) is the leading open source web content management platform (CMS) in the Microsoft ecosystem.
N2 CMS, an open source CMS for ASP.NET
Orchard is a free, open source, community-focused Content Management System built on the ASP.NET MVC platform.
Headless CMS and Content Managment Hub
The simple, flexible and friendly ASP.NET CMS used by more than 730.000 websites