use clear:left; or clear:both in your css.
#map { float:left; width:700px; height:500px; }
#list { float:left; width:200px; background:#eee; list-style:none; padding:0; }
#similar { float:left; width:200px; background:#000; clear:both; }
<div id="map"></div>
<ul id="list"></ul>
<div id ="similar">
this text should be below, not next to ul.
</div>