I recently stumbled across an open source C# and ASP.NET-based content management system called Umbraco that looks pretty nice. You download Umbraco here, or you can watch a few videos about it here. You can check out the main developer's blog here. To get the source, you've got to connect to their SVN repository, the details of which can be found here. Finally, you can check out a demo here.
Update: Changed the links due to the Umbraco site update.