Here is a small list of some content management systems written in ASP.NET. I’ll appreciate to notify me if you find more of them!

http://www.kentico.com/

http://www.visitmix.com/Lab/Oxite

http://umbraco.org/

http://www.axcms.net/

http://www.sitefinity.com/

http://n2cms.com/

http://www.dotnetnuke.com

http://www.telligent.com

http://kooboo.com/

http://www.mojoportal.com