using System;
using System.Diagnostics;
using System.Globalization;
using System.IO;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading;
using System.Web;
 
namespace WebMonitor {
    public class UnhandledExceptionModule: IHttpModule {
 
        static int _unhandledExceptionCount = 0;
 
        static string _sourceName = null;
        static object _initLock = new object();
        static bool _initialized = false;
        static string _mailServer = "mymailserver";
        static string _mailTo = "abc@xyz.com";
 
        public void Init(HttpApplication app) {
 
            // Do this one time for each AppDomain.
            if (!_initialized) {
                lock (_initLock) {
                    if (!_initialized) { 
 
                        _sourceName = string.Format(CultureInfo.InvariantCulture, "ASP.NET {0}.{1}.{2}.0",
							System.Environment.Version.Major, 
							System.Environment.Version.Minor,
							System.Environment.Version.Build);
 
                        if (!EventLog.SourceExists(_sourceName)) {
                            throw new Exception(String.Format(CultureInfo.InvariantCulture,
                                                              "There is no EventLog source named '{0}'.", 
                                                              _sourceName));
                        }
 
                        AppDomain.CurrentDomain.UnhandledException += new UnhandledExceptionEventHandler(OnUnhandledException);
 
                        _initialized = true;
 
						LogMessage("UnhandledExceptionModule started", "", EventLogEntryType.Information);
 
                    }
                }
            }
        }
 
        public void Dispose() {
        }
 
        void OnUnhandledException(object o, UnhandledExceptionEventArgs e) {
            // Let this occur one time for each AppDomain.
            if (Interlocked.Exchange(ref _unhandledExceptionCount, 1) != 0)
                return;
 
            StringBuilder message = new StringBuilder("\r\n\r\nUnhandledException logged by UnhandledExceptionModule.dll:\r\n\r\nappId=");
 
            string appId = (string) AppDomain.CurrentDomain.GetData(".appId");
            if (appId != null) {
                message.Append(appId);
            }
 
            Exception currentException = null;
            for (currentException = (Exception)e.ExceptionObject; currentException != null; currentException = currentException.InnerException) {
                message.AppendFormat("\r\n\r\ntype={0}\r\n\r\nmessage={1}\r\n\r\nstack=\r\n{2}\r\n\r\n",
                                     currentException.GetType().FullName, 
                                     currentException.Message,
                                     currentException.StackTrace);
            }           
 
			LogMessage("UnhandledExceptionModule exception", message.ToString(), EventLogEntryType.Error);
        }
 
		void LogMessage(string subject, string message, EventLogEntryType severity)
		{
			try
			{
				EventLog Log = new EventLog();
				Log.Source = _sourceName;
				Log.WriteEntry(subject + "\n" + message, severity);
			}
			catch  {}
			try
			{
				System.Web.Mail.MailMessage msg = new System.Web.Mail.MailMessage();
				msg.Body = message;
				msg.To = _mailTo;
				msg.Subject = severity.ToString() + ": " + subject;
				msg.From = _mailTo;
 
				System.Web.Mail.SmtpMail.SmtpServer = _mailServer;
				System.Web.Mail.SmtpMail.Send(msg);
			}
			catch {}
		}
 
    }
}

<system.web>
  <httpModules>
    <add type="WebMonitor.UnhandledExceptionModule, UnhandledExceptionModule" name="UnhandledExceptionModule"/>
  </httpModules>
</system.web>

using System.Threading;
Timer t;
private void btnTest_Click(object sender, System.EventArgs e)
{
  t = new Timer(new TimerCallback(t_Elapsed), null, 250, Timeout.Infinite);
}
private void t_Elapsed(object state)
{
  throw new Exception("My unhandled exception");
}