Free HTML Editor for ASP.Net

Here is a sample:
http://www.asp.net/ajax/ajaxcontroltoolkit/Samples/HTMLEditor/HTMLEditor.aspx

Have a look at here to see the tutorial :
http://www.asp.net/ajax/tutorials/how-do-i-use-the-html-editor-control--cs

You will need AjaxToolkit from CodPlex to be able use it.