I know this question has already been answered, but having dealt with layout a fair bit, I wanted to add an alternative answer that solves a few traditional problems with floating elements...
You can see the updated example in action here.
http://jsfiddle.net/Sohnee/EMaDB/1/
It makes no difference whether you are using HTML 4.01 or HTML5 with semantic elements (you will need to declare the left and right containers as display:block if they aren't already).
CSS
.left {
background-color: Red;
float: left;
width: 50%;
}
.right {
background-color: Aqua;
margin-left: 50%;
}
HTML
<div class="left">
<p>I have updated this example to show a great way of getting a two column layout.</p>
</div>
<div class="right">
<ul>
<li>The columns are in the right order semantically</li>
<li>You don't have to float both columns</li>
<li>You don't get any odd wrapping behaviour</li>
<li>The columns are fluid to the available page...</li>
<li>They don't have to be fluid to the available page - but any container!</li>
</ul>
</div>
There is also a rather neat (albeit newer) addition to CSS that allows you to layout content into columns without all this playing around with divs:
column-count: 2;