Stackoverflow open sourced their version of Markdown to the world. Its called MarkdownSharp and is written in C#.
Somebody wrote a HtmlHelper here:
http://blog.dantup.com/2011/03/an-asp-net-mvc-htmlhelper-extension-method-for-markdown-using-markdownsharp
If you are looking for how to implement a javascript editor there is an existing question:
Integrate Markitup text editor to ASP.NET MVC project