Umbraco: free, open source CMS

Umbraco - The open source ASP.NET CMS

even Microsoft is using it for running www.asp.net
and apparently some 110 000 other web sites

podcast Building Umbraco with Benjamin Howarth @ .NET Rocks

Umbraco - Introducing ContentService aka the v6 API

There are similar alternatives

Orchard, newer, based on ASP.NET MVC
also free and related to Microsoft people.

How Orchard works - Orchard Documentation

Oldest, based on WebForms

.NET CMS Software for Content Management | DNN Software


content management system - DotNetNuke, Umbraco or Orchard - Stack Overflow