http://www.umbraco.org
Umbraco is the fastest growing Web CMS on the Microsoft ASP.NET platform and the only open source Web CMS that'll put a smile on both designers, developers and editors. It's the most flexible CMS on the Microsoft platform, yet incredibly fast and easy to implement. It comes with a beautiful web client that works in any modern browser on any platform and gives you FULL CONTROL of both markup, content models and integration via .NET.