Q:

How to center list

If I have a <select> list on my html page:
<body>
<div id="side-bar">...</div>
<div id="list">
       <select id="mylist" size="10"></select>
</div>
</body>

How to CSS mylist to make it located in the middle of the <div id="list"> area? (Both horizontally and vertically)

A:

You could try #mylist {margin: 0 auto}. It centers only the select element. If you want to center all the content within your div then #list { text-align: center; } should do the trick.
EDIT:
To center the select tag both vertically and horizontally you can do like this:
<style type="text/css">
#list {
    width: 200px;
    height: 200px;
    background: #fa2;
}
#mylist {
    width: 100px;
    line-height: 20px;
    position: relative;
    top: 50%;
    left: 50%;
    margin: -10px 0 0 -50px;
}
</style>
<div id="list">
       <select id="mylist">
           <option value="1">test</option>
       </select>
</div>

