Código CSS

*{ margin:0; padding:0;}
body{
    background:#fff;
}

#menu{
    width:900px;
    height:34px;
    background:#DDD;
    margin:10px auto;
    border:1px solid #999;
    border-radius:3px;
}
#menu ul{
    list-style-type:none;
    float:left;
    width:100%;
}
#menu ul li{
    float:left;
    position:relative;
    font:15px "Trebuchet MS", Arial, Helvetica, sans-serif;
    padding:0 10px;
    line-height:36px;
    border-right:1px solid #999;
}
#menu ul li:hover{
    background:#CCC;
    border:1px solid #999
}
#menu ul li:hover ul{
    left:0;
}
#menu ul li a{
    text-decoration:none;
    color:#000;
}
#menu ul li ul{
    position:absolute;
    left:-999em;
    top:36px;
    width:160px;
    background:#CCC;
    border:1px solid #999;
}
#menu ul li ul li{
    width:160px;
    border-right:0px;
    background:#DDD;
}