Prueba con esto:
Código HTML:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>Documento sin título</title>
</head>
<style type="text/css">
#contenedor span { width:150px; float:left; }
.limpia { clear:both; }
</style>
<body>
<div id="contenedor">
<span>1</span>
<span>2</span>
<span>3</span>
<span>4</span>
<span>5</span>
<span>6</span>
<div class="limpia"></div>
</div>
</body>
</html>