The boxes at the left have associated CSS popups, with relevant styles:

#nav1 a .pop {
   display: block;
   position: absolute;
   top: 0; left: 13.2em;
   visibility: hidden;
}
#nav1 a:hover .pop {
   visibility: visible;
}

They work in IE5.5+/Win, Moz, Op7+, Saf, IE5/Mac
Don't work in IE5.0/Win, Op6

The boxes at the left have associated CSS popups, with relevant styles:

#nav2 a .pop {
   display: none;
}
#nav2 a:hover .pop {
   display: block;
   position: absolute;
   top: 0; left: 13.2em;
}

They work in IE5+/Win, Moz, Op7+, Saf, IE5/Mac
Don't work in Op6