http://wordpress.org/support/topic/inserting-ads-between-every-5th-post
http://wordpress.org/support/topic/how-to-show-code-after-x-amount-of-posts
http://wordpress.org/support/topic/easy-way-to-add-a-banner-between-every-x-number-of-posts
http://wordpress.org/support/topic/make-the-loop-do-something-after-every-x-post