You are telling jquery to slide the element down but never telling it to slide up again.
The hover
function will take a mouse in and mouse out parameter, try.
$(document).ready(function() {
$('.menu>li').hover(function(){
$('.menu>li>ul').stop(true,true).slideDown("slow");
}, // <-- this comma is important.
//the anonymous function after this will run on mouse out
function(){
$('.menu>li>ul').stop(true,true).slideUp("slow");
});
});