Add a wrapper around your UL and use the new CSS3 "columns":
<div class="columns">
<ul>
<li>Item 1</li>
<li>Item 2</li>
<li>Item 3</li>
<li>Item 4</li>
</ul>
</div>
And then style with CSS:
.columns {
-moz-column-count: 3;
-moz-column-gap: 1em;
-webkit-column-count: 3;
-webkit-column-gap: 1em;
}
Here's a jsFiddle: http://jsfiddle.net/NEHwE/