There are a fair few CMS solutions floating around in .NET at the moment, a good general comparison tool can be found at CmsMatrix.org.
Cuyahoga
In the past, the Cuyahoga Website Framework has looked always fairly interesting, it is also built on NHibernate which to me is a plus. Most things I've done with NHibernate have generally "just worked", maybe it's because NHibernate development is pounded with unit tests, whatever it is, it works.
I haven't had any experience developing any extensions for Cuyahoga, so I don't know exactly how extensible it is yet. In terms of usability, I did find the interface...