insert advertisement block within views list
You can override the format specific template like views-view-unformatted.tpl.php (you can find the right file based on your requirement) and then use some custom code like 

  &lt;?php if (($id % 2) == 0): ?&gt;
    Do what-ever you want to do here. 
  &lt;?php endif; ?&gt;


This module hosted on github https://github.com/pedroposada/custom_views_nthrow seems to do the same more modularly. 

Instructions : http://www.pedroposada.com/blog/insert-whatever-every-nth-row-views.html-0