From http://pmob.co.uk/pob/centred-float.htm:
The premise is simple and basically just involves a widthless float wrapper that is floated to the left and then shifted off screen to the left width position:relative; left:-50%. Next the nested inner element is reversed and a relative position of +50% is applied. This has the effect of placing the element dead in the center. Relative positioning maintains the flow and allows other content to flow underneath.
Code
#buttons{
float:right;
position:relative;
left:-50%;
text-align:left;
}
#buttons ul{
list-style:none;
position:relative;
left:50%;
}
#buttons li{float:left;position:relative;}/* ie needs position:relative here*/
#buttons a{
text-decoration:none;
margin:10px;
background:red;
float:left;
border:2px outset blue;
color:#fff;
padding:2px 5px;
text-align:center;
white-space:nowrap;
}
#buttons a:hover{ border:2px inset blue;color:red;background:#f2f2f2;}
#content{overflow:hidden}/* hide horizontal scrollbar*/
<div id="buttons">
<ul>
<li><a href="#">Button 1</a></li>
<li><a href="#">Button 2's a bit longer</a></li>
<li><a href="#">Butt 3</a></li>
<li><a href="#">Button 4</a></li>
</ul>
</div>