Umbraco
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.
Umbraco sites showcase, 1720 sites using Umbraco
