El css que genera el 100% height:

.contenedor{
overflow:hidden;
width:680px;
margin:0 auto;
}
.left{
float:left;
width:100px;
padding:20px;
padding-bottom:9999em;
margin-bottom:-9999em;
background:#555555;
}
.main{
float:left;
width:500px;
padding:20px;
background:#bbbbbb;
}