Versatile ASP.NET CMS in C# [closed]
Umbraco - http://umbraco.org
Written entirely in C#, free, small footprint, uses master pages and very flexible.
Custom .NET can be added via Macros (.NET user controls) or complete ASPX pages, provided you specify in the web.config that the file exists and Umbraco doesn't try to find it in the content database.
N2 is a free open source ASP.NET CMS with support also for MVC. It's opinionated, uses .Net classes for the programming model and is very open and extensible.