Код:
<div style="border:1px solid red;">
<div style="float:left;height:200px;left:30px;margin:10px -100% 0 0;position:relative;z-index:1;border:1px solid #000;background:pink;">leftbar</div>
<div style="height:100px;border:1px solid blue;background:yellow;margin:0;">main</div>
<div style="clear:both;">footer</div>
</div>