Excellent Open Source DOT net/ .net CMS

Dotnetnuke
: the most widely adopted Web Content Management Platform for building
web sites and web applications on Microsoft .NET. Organizations use
DotNetNuke to quickly develop and deploy interactive and dynamic web
sites, intranets, extranets and web applications. The DotNetNuke
platform is available in a free Community and subscription-based
Professional, Elite and Elite Premier Editions. The open platform
allows DotNetNuke to function as both a content management system (CMS)
and application development framework. DotNetNuke allows your
organization to leverage your existing Microsoft investments to create
rich, highly interactive web sites and web applications.

Mojoportal
: You can think of mojoPortal as a Starter Kit for Advanced ASP.NET Web
Sites and Portals.  Out of the box, mojoPortal provides dynamic
database driven web site(s) with an XHTML compliant Content Management
System and community features like Blogs, Forums, Image Galleries, etc.

Umbraco : Umbraco
makes it quick, easy and fun to develop web applications. Umbraco
features an easy to use framework for working with dynamic content and
javascript called umbraco Base. The umbraco Base framework enables
developers to create simple urls to umbraco content and .NET resources,
making it much easier to work with .NET-based data directly from any
javascript framework, like jQuery, YUI, Prototype or whatever library
you prefer to use.

Cuyahoga
: Cuyahoga is an open source .NET web site framework. It provides
content management capabilities and has a modular approach. Currently
it works with MS .NET 2.0-3.5 with SQL Server, PostgreSQL or MySQL as
database backend. Also, Mono is fully supported.

N2CMS : For those
looking for an out of the box experience N2 CMS contains a package of
functional templates with News, Wiki, Photo Galleries, FAQs, RSS, Data
Entry, Polls and more. With these you’ll be up and running no time
(demo). The functional templates are completly optional. If you want
you can just use an adaptive editing interface that integrates quite
nicely into any ASP.NET code. Features include full control of content
and nodes, drag & drop, versioning, wizards, export/import,
security, globalization and more.

Thebeerhouse
: TheBeerHouse is a website developed with Microsoft’s MVC Framework
which includes a number of features and modules that you expect from a
typical CMS / e-commerce website, such as: Membership system with
registration, login, profile setup and complete administration, Content
management system for publishing and syndicating articles and photos,
with support for categories, comments, rating and profile-based
personalization, Opinion polls with support for multiple active polls
and archived polls etc.

Kooboo :
Kooboo CMS allows you to create a professional website efficiently
without losing out on product quality. Based on the ASP.NET MVC system
that you are familiar with, and our easy-to-use tutorials and quick
response forum provide clear instruction and support to give you
confidence to unleash your creative potential.

Let us know if you have used any other Asp.Net CMS which is Open
Source and has some great options to fill in this list. I will surely
add your input.