There’s a reason that most websites run on Linux servers, and why so many run on PHP with MySQL server. That reason is price. MySQL is free, PHP is free, Linux is relatively free depending on which flavour you use.
The Microsoft platform has, for several years, been an expensive one but the Umbraco content management system (CMS) is a bit of a game changer. This CMS is an open source CMS based on Microsoft’s ASP.NET that even allows editing with Microsoft Word. With Umbraco, designers can create accessible and valid xhtml with their mark-up left intact.