Source: http://www.kriesi.at/archives/create-a-multilevel-dropdown-menu-with-css-and-improve-it-via-jquery

<!-- HTML for a nice gentle Menu for any website -->
<nav>
    <ul class="menu">
    	<li><a href="#">Menu - Level 1</a></li>
        <li><a href="#">Menu - Level 1</a>
            <ul>
            	<li><a href="#">Menu - Level 2</a></li>
                <li><a href="#">Menu - Level 2</a>
                    <ul>
                    	<li><a href="#">Menu - Level 3</a></li>
                	<li><a href="#">Menu - Level 3</a></li>
                    </ul>
                </li>
            </ul>
        </li>
        <li>Menu - Level 1</li>
    </ul>
</nav>

/* CSS CODE FOR A MENU, A NICE ONE. AUTHOR: Forgotten. BUT HE/SHE CREATED A NICE ONE. THANKS OBVIOUSLY TO HIM/HER */

.menu, .menu ul{
margin:0;
padding:0;
list-style-type:none;
list-style-position:outside;
position:relative;
line-height:1.5em; 
}

.menu a{
display:block;
padding:0px 5px;
border:1px solid #CCC;
color:#fff;
text-decoration:none;
background-color:#CCC;
}

.menu a:hover{
background-color:#fff;
color:#333;
}

.menu li{
float:left;
position:relative;
}

.menu ul {
position:absolute;
display:none;
width:12em;
top:1.5em;
}

.menu li ul a{
width:12em;
height:auto;
float:left;
}

.menu ul ul{
top:auto;
}	

.menu li ul ul {
left:12em;
margin:0px 0 0 10px;
}

.menu li:hover ul ul, .menu li:hover ul ul ul, .menu li:hover ul ul ul ul{
display:none;
}

.menu li:hover ul, .menu li li:hover ul, .menu li li li:hover ul, .menu li li li li:hover ul{
display:block;
position:absolute;
z-index:5000;
}