C#

[DllImport("shell32.dll")]
static extern int SHGetFolderLocation(IntPtr hwndOwner, int nFolder,
  IntPtr hToken, uint dwReserved, IntPtr ppidl);

const int WM_APP = 0x8000;
const int WM_DO_SOMETHING_MAN = WM_APP + 1; // Your custom message ID.

// Get the PostMessage() API function:
[DllImport("User32.Dll")]
private static extern System.Int32 PostMessage(int hwnd, int msg, int lparam, int wparam);

// Your mutex:
private static Mutex AppMutex = new Mutex(true, "TestApp");

// The form's constructor:
public FormMain()
{
   InitializeComponent();

   // This is important; without it, OnNotifyMessage() is inert:
   SetStyle( ControlStyles.EnableNotifyMessage, true );
}

// The app's entrypoint:
[STAThread]
static void Main() 
{
   if(AppMutex.WaitOne(0, false))
   {
      Application.Run(new FormMain());
      AppMutex.ReleaseMutex();
   }
   else
   {
      // Look through the processes to find "the other one" that
      // is running and make sure this instance doesn't accidentally
      // find itself!
      Process thisProcess = Process.GetCurrentProcess();
      Process [] allProcesses = System.Diagnostics.Process.GetProcesses();
      foreach( Process process in allProcesses )
      {
         if( process.MainModule.FileName == Application.ExecutablePath )
         {
            if(process.Id != thisProcess.Id) // Not me?
            {
               int hwnd = process.MainWindowHandle.ToInt32();
               PostMessage( hwnd, WM_DO_SOMETHING_MAN, 0, 0 );
               break;
               // There is an itsy bitsy corner case bug here, if somehow
               // three or more instances started virtually simultaneously...
            }
         }
      }
   }
}

// The handler for the custom message:
protected override void OnNotifyMessage(Message m)
{
   if( m.Msg == WM_DO_SOMETHING_MAN )
      DoSomethingClever();  // Pop your window to the top, or whatever.
}