There are a number of open source asp.net CMS around, such as n2 and mojoportal. Even our company build an opensource CMS, AdvGenCMS. That is still in beta status. We just started to rolling out for some customers to test. Thus, we are still using some opensource cms. There are two of them I like most, Umbraco and Orchard. I love Orchard a lot, it is using MVC, Fluent Hibernate and Autofac. Moreover, the admin interface has a lot of functions and you can drag and drop the widget. That is easy to use. The major problem used too many opensource libraries. I need the system to be simpler.
So, I come to the second choice, Umbraco. This is widely supported, a number of companies are using this.
Moreover, the admin interface is very simple, nice and clean, that is a out of box solution!