Disclaimer: I'm working for this company.

As I see, the question is old but maybe someone search solution for this too. Our component RTF to HTML allows to convert RTF to HTML. You may download a component or try online-demo. Try the trial version first if you have a doubt. :) Trial is free.

Here's the code sample for the converting from RTF to HTML in ASP.NET:

    SautinSoft.RtfToHtml r = new SautinSoft.RtfToHtml();
    r.OutputFormat = SautinSoft.RtfToHtml.eOutputFormat.HTML_401;
    r.ImageStyle.IncludeImageInHtml = false; //To save images inside HTML as binary data specify this property to 'true'

    r.ImageStyle.ImageFolder = Server.MapPath(""); 
    r.ImageStyle.ImageSubFolder = "images";
    r.ImageStyle.ImageFileName = "picture";       

    string rtf = ".....";
    string html = r.ConvertString(rtf);        

    //show HTML
    if (html.Length>0)
    {
        Response.Buffer = true;
        Response.Clear();
        Response.ContentType = "text/html";
        Response.Write(html);
        Response.Flush();
        Response.End();
    }

TL;DR: I recommend using the OpenXml format and the HtmlToOpenXml nuget package if possible.

Microsoft Word COM

I haven't really searched much into this topic as a my use case is to use the functionality on a server which makes COM components not a great selection.

XHTML2RTF

As @IAmTimCorey mentioned you can use this codeproject library.

Disadvantages are:

• Limited supported HTML and CSS
• Not really .NET
• ...

Windows Forms Web Browser

As @Jerry mentioned you can use the Windows Forms WebBrowser control.

Disadvantages are:

• Reference to System.Windows.Forms
• Uses copy & paste (problematic for multithreading)
• Only works in an STA thread

Not supported features include:

• Fonts
• Colors
• Numbered lists
• Strikethrough (del element)
• ...

DevExpress

Code sample of "Paul V" from the devexpress support center. (03.02.2015)

public String ConvertRTFToHTML(String RTF)
{   
    MemoryStream ms = new MemoryStream();
    StreamWriter writer = new StreamWriter(ms);
    writer.Write(RTF);
    writer.Flush();
    ms.Position = 0;
    String output = "";
    HtmlEditorExtension.Import(HtmlEditorImportFormat.Rtf, ms, (s, enumerable) => output = s);

    return output;
}

public String ConvertHTMLToRTF(String Html)
{
    MemoryStream ms = new MemoryStream();
    var editor = new ASPxHtmlEditor { Html = html };

    editor.Export(HtmlEditorExportFormat.Rtf, ms);

    ms.Position = 0;
    StreamReader reader = new StreamReader(ms);

    return reader.ReadToEnd();
}

Or you could use the RichEditDocumentServer type as shown in this example.

• A license for devexpress can cost from around 1500.- USD to 2200.- USD.

Unknown what actually is supported.

Disadvantages are:

• Price
• Quite a lot of references for one small thing
• More?

Not supported features include:

• Striketrough (del element)

Sautinsoft

public string ConvertHTMLToRTF(string html)
{
    SautinSoft.HtmlToRtf h = new SautinSoft.HtmlToRtf();
    return h.ConvertString(htmlString);
}

public string ConvertRTFToHTML(string rtf)
{
    SautinSoft.RtfToHtml r = new SautinSoft.RtfToHtml();
    byte[] bytes = Encoding.ASCII.GetBytes(rtf);
    r.OpenDocx(bytes );
    return r.ToHtml();
}

More examples and configuration options can be found here and here.

• A licence for this component can cost from 400.- USD to 2000.- USD.

Supported is the following:

• HTML 3.2
• HTML 4.01
• HTML 5
• CSS
• XHTML

Disadvantages are:

• I'm not sure how active the development is
• Price

Usage knowledgebase:

• Converting numbered lists from the trix angular editor destroys indend

DIY

If you only wanted to support limited functionality you could write your own converter. I would not recommend this if the supported feature set is too large. (Sautinsoft claims to have written over 20'000 lines of code).

I have a small sample project here but is only for educational purposes in its current state.

OpenXml

If the OpenXml format is also ok for your use case you can use the HtmlToOpenXml nuget package. Its free and did support all features I've tested the other solutions against.

The project is based on the Open Xml SDK by microsoft and seems active.

public static byte[] ConvertHtmlToOpenXml(string html)
{
    using (var generatedDocument = new MemoryStream())
    {
        using (var package = WordprocessingDocument.Create(generatedDocument, WordprocessingDocumentType.Document))
        {
            var mainPart = package.MainDocumentPart;
            if (mainPart == null)
            {
                mainPart = package.AddMainDocumentPart();
                new Document(new Body()).Save(mainPart);
            }

            var converter = new HtmlConverter(mainPart);
            converter.ParseHtml(html);

            mainPart.Document.Save();
        }

        return generatedDocument.ToArray();
    }
}

• Link to example gist