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

#nav1 a .pop {
	display: block;
	position: absolute;
	top: auto;
	visibility: hidden;
}
#nav1 a:hover .pop {
	visibility: visible;
}

They work in IE5.5+/Win, Moz, Saf, Op7+, 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: auto;
}

They work in IE5+/Win, Moz, Saf, Op8+, IE5/Mac
Don't work in Op6; rendering problems in Op7