Статьи и справочные
материалы

<style type="text/css">
   #block-one, #block-two {
      width: 300px;
      border: 1px solid #999;
      padding: 5px;
   }
   #block-one { float: left; }
   #block-two { float: right; }
</style>
<div id="block-one">
   Lorem ipsum dolor sit amet, consectetuer adipiscing elit, sed diam nonummy nibh
   euismod tincidunt ut laoreet dolore magna aliquam erat volutpat.
</div>
<div id="block-two">
   Duis autem vel eum iriure dolor in hendrerit in vulputate velit esse molestie
   consequat, vel illum dolore eu feugiat nulla facilisis at vero eros et accumsan
   et iusto odio dignissim qui blandit praesent luptatum zzril delenit augue duis
   dolore te feugait nulla facilisi.
</div>

<script type="text/javascript">
var oldHandler = window['onload']; // сохраняем старый onload (если есть)
window['onload'] = function() {
   if(typeof(oldHandler) == 'function') {
      oldHandler();
   }
   setHeight(); // наша функция
};
function setHeight() {
   var b1 = document.getElementById('block-one');
   var b2 = document.getElementById('block-two');
   if (!b1 || !b2) return;
   var h1 = b1.offsetHeight;
   var h2 = b2.offsetHeight;
   if (h1 > h2) {
      b2.style.height = (h1 - 10) + 'px';
      b1.style.height = (h1 - 10) + 'px';
   }else if (h1 < h2) {
      b1.style.height = (h2 - 10) + 'px';
      b2.style.height = (h2 - 10) + 'px';
   }
}
</script>