Rainbow Portal 
The best .NET CMS so far. Now that .NET 2.0 version has been released, it got even better, being able to upgrade your .NET 1.1 portals to new 2.0 version. This websites runs Rainbow Portal. It's multilanguage, blazing fast and easy to extend. While 1.x version was licensed under GPL, this new version will be released under BSD so it will be easier to develop new systems based on RP.
DotNetNuke
For sure, most famous .NET CMS. It has larger .NET community and tons of modules, even if most of them are commercial. It is easy to extend and has some nice features, expecially in UI area. However, it's not that fast (RP is way faster), it's not multilanguage (even if there are a few modules for that) plus it tends to consume much memory. Still is probably best choice for a .NET CMS, if you want to be part of a large community. Licensed under BSD.
MyWebPages Starter Kit
A new CMS system developed by Microsoft as a starter kit to let developers start on a working solution. It basically includes all ASP.NET 2.0 features for Web development, from Master pages, to sitemaps, membership APIs, roles and so on. It also features a very good module system to let developers extended plus it doesn't require a database backend since it's based on XML files. It's not localized so you must use additional hacks to provide ML contents. Other than that, it's a good system to build your applications upon and it's released under MS-PL, which is basically BSD-like. It also lacks some features which are part of more mature systems but MWPSK has a fair set of initial modules for basic portals. Since it doesn't need to support legacy solutions from elder .NET 1.x releases, it's very easy to extend and to build upon so it can be your first choice to have a simple system to customize and provide additional code.
Cuyahoga
A relatively new CMS system based NHibernate. It supports major SQL backends like MS SQL, MySQL and PostgreSQL, which is something you don't find so easily in .NET arena. Didn't play with this, actually, so I'm only listing it (and that's why I don't have any rating, to be fair to developers). It looks a pretty interesting project, expecially in modules (which can communicate with each other across pages, according to developers), a flexible layout engine and support for ASP.NET 2.0. If I will have more time to evaluate it, I will be post more details. Licensed under BSD.