.stop( [clearQueue ] [, jumpToEnd ] )
var sub_menu = $('.main-menu .sub-menu');//select all sub menus
$('.main-menu > ul > li').hover(function () {
sub_menu.stop(true, true); //use stop on all submenus
$(this).find('.sub-menu').slideDown('slow');
}, function () {
sub_menu.stop(true, true);//use stop on all submenus
$(this).find('.sub-menu').slideUp('slow');
});