All popup windows, but the only users I can see offhand are balloon-help and floating-toolbar.  Oh, and dialog boxes, probably (I have those turned off).
Robert
-- 
2006-12-22    <
rpluim@gmail.com>
    * frame-msw.c (mswindows_init_frame_1): Honour 'initially-unmapped.
    (mswindows_init_frame_3): ditto.
pcl-cvs: descending directory src/
Index: frame-msw.c
===================================================================
RCS file: /pack/xemacscvs/XEmacs/xemacs/src/frame-msw.c,v
retrieving revision 1.59
diff -u -r1.59 frame-msw.c
--- frame-msw.c    2005/11/25 01:42:02    1.59
+++ frame-msw.c    2006/12/22 08:39:03
@@ -151,6 +151,16 @@ 
   /* Pick up relevant properties */
   initially_unmapped = Fplist_get (props, Qinitially_unmapped, Qnil);
+
+  if (!NILP(initially_unmapped))
+    {
+      f->visible = 0;
+    }
+  else
+    {
+      f->visible = 1;
+    }
+  
   name = Fplist_get (props, Qname, Qnil); 
   popup = Fplist_get (props, Qpopup, Qnil);
@@ -305,7 +315,8 @@
 mswindows_init_frame_3 (struct frame *f)
 {
   /* Don't do this earlier or we get a WM_PAINT before the frame is ready */
-  ShowWindow (FRAME_MSWINDOWS_HANDLE (f), SW_SHOWNORMAL);
+  if (f->visible == 1)
+    ShowWindow (FRAME_MSWINDOWS_HANDLE (f), SW_SHOWNORMAL);
   SetForegroundWindow (FRAME_MSWINDOWS_HANDLE (f));
   DragAcceptFiles (FRAME_MSWINDOWS_HANDLE (f), TRUE);
 }