HttpHandler that helps respect the Request flow and serves the default documents of a directory instead of allowing e.g. Umbraco to kidnap the request for there purposes.

DefaultDocumentsHttpModule.cs

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Web;

/// <summary>
/// Respect the DefaultDocuments even when Umbraco takes over the HttpModule pipeline
/// Add file to App_Code (or compile with your project) and 
/// Add to web.config on system.web > httpModules add name="MyDefaultDocumentsHttpModule" type="DefaultDocumentsHttpModule" 
/// Add to web.config on system.webServer > modules add name="MyDefaultDocumentsHttpModule" type="DefaultDocumentsHttpModule" 
/// </summary>
public class DefaultDocumentsHttpModule : IHttpModule
{
    #region IHttpModule Members
    public void Init(HttpApplication context)
    {
        context.BeginRequest += ProcessRequest;
    }

    public void Dispose() { }
    #endregion

    protected void ProcessRequest(object sender, EventArgs e)
    {
        DefaultDocuments();
    }

    static void DefaultDocuments()
    {
        HttpContext context = HttpContext.Current;
        HttpRequest request = context.Request;

        var lastsegment = request.Url.Segments.LastOrDefault();
        if (lastsegment != null && !lastsegment.Contains("."))
        {
            if (!IsDirectory(request.MapPath(request.Path))) return;

            var separator = lastsegment.EndsWith("/") ? "" : "/";
            var root = request.Path + separator;
            
            if (IsFile(request.MapPath(root + "default.htm"))) // Hardcoded for avoiding to many IO operations e.g. reading settings
            {
                if (separator == "/") context.Response.Redirect(root,true); // Required for correct handling of relative page content /xxx is not the same content as /xxx/
                context.RewritePath(root + "default.htm");
                return;
            }
            if (IsFile(request.MapPath(root + "default.asp")))
            {
                if (separator == "/") context.Response.Redirect(root,true);
                context.RewritePath(root + "default.asp");
                return;
            }
            if (IsFile(request.MapPath(root + "index.html")))
            {
                if (separator == "/") context.Response.Redirect(root,true);
                context.RewritePath(root + "index.html");
                return;
            }
        }
        return;
    }

    private static bool IsFile(string mapPath)
    {
        return System.IO.File.Exists(mapPath);
    }

    private static bool IsDirectory(string mapPath)
    {
        return System.IO.Directory.Exists(mapPath);
    }
}