If anyone follows me on Twitter, you'll have seen my early morning rant a few weeks ago about the woeful state of CMS on the .NET platform, with particular reference to Open Source offerings. This was sparked off by the current depressing thread on the Telligent GraffitiCMS forum about the future of the product.
If you look in the non-Microsoft world there are some quite well established (Open Source), mature systems, each with their own rich eco-systems; Plone, Joomla, Drupal are just three examples. James Saull sent me a link to CMS Watch's "CMS vendor tube map" - a quite impressive distillation of the entire CMS universe:

Source - CMS Watch.
If you delve into the non-Open Source CMS market on the Microsoft Platform, there are some very strong contenders:
But these were a tad difficult to find as the Open Source Microsoft Platform, doesn't suffer from a drought of Blogging Platforms:
After doing a bit of digging, the top Open Source .NET CMS Platforms are:
Umbraco seems to be the most popular and is gaining the most momentum (version 4 has ~30,000 downloads on CodePlex) and offers a paid for version that comes with support and some extra tooling, there is even a yearly conference. Cuyahoga is a bit of an unknown entity, although Oren (Ayende) seems to rate it highly.
N2 is an impressive entry - in that it takes a different approach to CMS; with Umbraco, you are forced to code within the Umbraco framework and thus create a CMS System with bespoke application functionality, whereas, N2 allows you to integrate CMS at the API Level, and thus allows you to create rich applications with CMS functionality. A subtle difference, but very powerful.