.show( time ) method
jsBin demo
$('a.post-tag').hover(function(){
var $this = $(this);
var offset = $this.offset();
var myPos = {X: offset.left, Y:offset.top+26};
$('#tag-menu').css({left:myPos.X, top:myPos.Y, width:300, height:200}).show(400);
// ^^^TADA !!
}, function(){
$('#tag-menu').stop(1,1).hide();
});