Mine

using System;
using System.Net;
using System.IO;
using System.Net.Sockets;

class X {
        static void Main ()
        {
                HttpListener l = new HttpListener ();
                l.Prefixes.Add ("http://localhost:8081/");
                l.Start ();
                l.BeginGetContext (callback, l);

                Console.WriteLine ("Now hit this with curl, like this: curl -H 'Content-Type: text/xml' -d @mindtouch.host/show-headers.startup.xml http://localhost:8081/");

                Console.ReadLine ();
        }

        static void callback (IAsyncResult ar)
        {
                Console.WriteLine ("Got a connection");
                HttpListener l = (HttpListener) ar.AsyncState;

                HttpListenerContext c = l.EndGetContext (ar);
                HttpListenerRequest request = c.Request;

                StreamReader r = new StreamReader (request.InputStream);
                r.ReadToEnd ();
                c.Response.OutputStream.WriteByte (65);
                c.Response.OutputStream.Flush ();
                c.Response.Close ();

                Console.WriteLine ("Done");
                l.Close ();
        }
}